This post has been updated. It was originally published Jan. 11, 2013.

Jan. 11, 2013: This post has been corrected.

You might have heard about the “kill list.” You’ve certainly heard about drones. But the details of the U.S. campaign against militants in Pakistan, Yemen, and Somalia -- a centerpiece of the Obama administration’s national security approach – remain shrouded in secrecy. Here’s our guide to what we know—and what we don’t know.

Where is the drone war? Who carries it out?

Drones have been the Obama administration’s tool of choice for taking out militants outside of Iraq and Afghanistan. Drones aren’t the exclusive weapon – traditional airstrikes and other attacks have also been reported. But by one estimate, 95 percent of targeted killings since 9/11 have been conducted by drones. Among the benefits of drones: they don’t put American troops in harm’s way.

The first reported drone strike against Al Qaeda happened in Yemen in 2002. The CIA ramped up secret drone strikes in Pakistan under President George W. Bush in 2008. Under Obama, they have expanded drastically there and in Yemen in 2011.

The CIA isn’t alone in conducting drone strikes. The military has acknowledged “direct action” in Yemen and Somalia. Strikes in those countries are reportedly carried out by the secretive, elite Joint Special Operations Command. Since 9/11, JSOC has grown more than tenfold, taking on intelligence-gathering as well as combat roles. (For example, JSOC was responsible for the operation that killed Osama Bin Laden.)

The drone war is carried out remotely, from the U.S. and a network of secret bases around the world. The Washington Post got a glimpse – through examining construction contracts and showing up uninvited – at the base in the tiny African nation of Djibouti from which many of the strikes on Yemen and Somalia are carried out. Earlier this year, Wired pieced together an account of the war against Somalia’s al-Shabaab militant group and the U.S.’s expanded military presence throughout Africa.

The number of strikes in Pakistan has ebbed in recent years, from a peak of more than 100 in 2010, to an estimated 46 last year. Meanwhile, the pace in Yemen picked up, with more than 40 last year. But there have been seven strikes in Pakistan in the first ten days of 2013.

Drone War Jargon AUMF The Authorization for Use of Military Force, an act of Congress passed days after the 9/11 attacks, giving the president authority to take "all necessary and appropriate force" against anyone involved in the attack or harboring those who were. Both Bush and Obama have claimed broad authorities to detain and kill terror suspects based on the AUMF. AQAP Al Qaeda in the Arabian Peninsula is the Yemen-based al Qaeda affiliate tied to the attempted Christmas Day airplane bombing in 2009. Over the past year, the U.S. has ramped up strikes against AQAP, targeting leaders as well as unspecified militants. Disposition Matrix A system for tracking terror targets and assessing when – and where – they could be killed or captured. The Washington Post reported this fall that the Disposition Matrix is an attempt to codify for the long haul the administration's "kill lists." Glomar A response rejecting a request for information on a classified program asserting that the information's mere existence can neither be confirmed nor denied. The name comes from 1968, when the CIA told journalists it could neither "confirm nor deny" the existence of a ship called the Glomar Explorer. The CIA has responded to information requests about its drone program with Glomar responses. JSOC Joint Special Operations Command is a secretive, elite segment of the military. JSOC squads carried out the Bin Laden raid and run the military's drone programs in Yemen and Somalia and also conduct intelligence gathering. Personality Strike A targeted attack on a particular individual identified as a terrorist leader. Signature Strike A strike against someone believed to be a militant whose identity isn't necessarily known. Such strikes are reportedly based on a "pattern of life" analysis – intelligence on their behavior suggesting that an individual is a militant. The policy, reportedly begun by Bush in Pakistan in 2008, is now allowed in Yemen, under stricter criteria. TADS Terror Attack Disruption Strikes, sometimes used to refer to some strikes when the identity of the target is not known. Administration officials have said that the criteria for TADS are different from signature strikes, but it is not clear exactly how.

How are targets chosen?

A seriesofarticles based largely on anonymous comments from administration officials have given partial picture of how the U.S. picks targets and carries out strikes. Two recent reports – from researchers at Columbia Law School and from the Council on Foreign Relations– also give detailed overviews of what’s known about the process.

The CIA and the military have reportedly long maintained overlapping “kill lists.” According to news reports last spring, the military’s list was hashed out in Pentagon-run interagency meetings, with the White House approving proposed targets. Obama would authorize particularly sensitive missions himself.

This year, the processreportedly changed, to concentrate the review of individuals and targeting criteria in the White House. According to the Washington Post, the reviews now happen at regular interagency meetings at the National Counterterrorism Center. Recommendations are sent to a panel of National Security Council officials. Final revisions go through White House counterterror adviser John Brennan to the president. Several profileshave highlighted Brennan’s powerful and controversial role in shaping the trajectory of the targeted killing program. This week, Obama nominated Brennan to head the CIA.

At least some CIA strikes don’t have to get White House signoff. The director of the CIA can reportedly green-light strikes in Pakistan. In a 2011 interview, John Rizzo, previously the CIA’s top lawyer, said agency attorneys did an exhaustive review of each target.

According to the Washington Post, the Obama administration's recent effort to impose more stringent requirements for kill lists and signature strikes exempts the CIA's campaign in Pakistan. The CIA will have at least a year to continue strikes in Pakistan according to its own protocols.

Doesn’t the U.S. sometimes target people whose names they don’t know?

Yes. While administration officials often have frequently framed drone strikes as going after “high-level al Qaeda leaders who are planning attacks” against the U.S., many strikes go after apparent militants whose identities the U.S. doesn’t know. The so-called “signature strikes” began under Bush in early 2008 and were expanded by Obama. Exactly what portion of strikes are signature strikes isn’t clear.

At various points the CIA’s use of signature strikes in Pakistan in particular have causedtensions with the White House and State Department. One official told the New York Times about a joke that for the CIA, “three guys doing jumping jacks,” was a terrorist training camp.

In Yemen and Somalia, there is debate about whether the militants targeted by the U.S. are in fact plotting against the U.S. or instead fighting against their own country. Micah Zenko, a fellow at the Council on Foreign Relations who has been critical of the drone program, toldProPublica that the U.S. is essentially running “a counterinsurgency air force” for allied countries. At times, strikes have relied on local intelligence that later proves faulty. The Los Angeles Times recently examined the case of a Yemeni man killed by a U.S. drone and the complex web of allegiances and politics surrounding his death.

How many people have been killed in strikes?

The precise number isn’t known, but some estimates peg the total around 3,000.

A number of groups are tracking strikes and estimating casualties:

· The Long War Journal covers Pakistan and Yemen.

· The New America Foundation covers Pakistan.

· The London Bureau of Investigative Journalism covers Yemen, Somalia, and Pakistan, as well as statistics from on drone strikes carried out in Afghanistan.

How many of those killed are have been civilians?

It’s impossible to know.

There has been considerable back-and-forth about the tally of civilian casualties. For instance, the New America Foundation estimates between 261 and 305 civilians have been killed in Pakistan; The Bureau of Investigative Journalism gives a range of 475 - 891. All of the counts are much higher than the very low numbers of deaths the administration claims. (We’ve detailedinconsistencies even within those low estimates.) Some analyses show that civilian deaths have dropped proportionally in recent years.

The estimates are largely compiled by interpreting news reports relying on anonymous officials or accounts from local media, whose credibility may vary. (For example, the Washington Post reported last month that the Yemeni government often tries to conceal the U.S.’ role in airstrikes that kill civilians.)

The controversy has been compounded by the fact that the U.S. reportedly counts any military-age male killed in a drone strike as a militant. An administration official told ProPublica, “If a group of fighting age males are in a home where we know they are constructing explosives or plotting an attack, it's assumed that all of them are in on that effort.” It’s not clear what if any investigation occurs after the fact.

Columbia Law School conducted an in-depth analysis of what we know about the U.S.’s efforts to mitigate and calculate civilian casualties. It concluded that the drone war’s covert nature hampered accountability measures taken in traditional military actions. Another report from Stanford and NYU documented “anxiety and psychological trauma” among Pakistani villagers.

This fall, the U.N. announced an investigation into the civilian impact – in particular, allegations of “double-tap” strikes, in which a second strike targets rescuers.

Why just kill? What about capture?

Administration officials have said in speeches that militants are targeted for killing when they pose an imminent threat to the U.S. and capture isn’t feasible. But killing appears to be is far more common than capture, and accounts of strikes don’t generally shed light on “imminent” or “feasible.” Cases involving secret, overseas captures under Obama show the political and diplomatic quandaries in deciding how and where a suspect could be picked up.

This fall, the Washington Post described something called the “disposition matrix” – a process that has contingency plans for what to do with terrorists depending where they are. The Atlantic mapped out how that decision-making might happen in the case of a U.S. citizen, based on known examples. But of course, the details of the disposition matrix, like the “kill lists” it reportedly supplants, aren’t known.

What’s the legal rationale for all this?

Obama administration officials have given a series of speeches broadly outlining the legal underpinning for strikes, but they never talk about specific cases. In fact, they don’t officially acknowledge the drone war at all.

The White House argues that Congress’ 2001 Authorization for Use of Military Force as well as international law on nations’ right to self-defense provides sound legal basis for targeting individuals affiliated with Al Qaeda or “associated forces,” even outside Afghanistan. That can include U.S. citizens.

“Due process,” said Attorney General Eric Holder said in a speech last March, “takes into account the realities of combat.”

What form that “due process” takes hasn’t been detailed. And, as we’vereported, the government frequently clams up when it comes to specific questions – like civilian casualties, or the reasons specific individuals were killed.

NBC News obtained a Justice Department memo that was given to some members of Congress in June laying out the administration's legal case for targeted killing in more detail. The memo, which was not classified, says that a U.S. citizen who is a "senior operational leader of al-Qaida or an associated force" can be targeted even if they are not tied to an active plot against the U.S. It also offers more detail on criteria for determining that capture is not feasible.

A federal judge had ruled earlier this year that the government did not have to release a separate secret legal memo making the specific case for the killing of Anwar al-Awlaki, a U.S. citizen. The judge also ruled the government did not have to respond to other requests seeking more information about targeted killing in general. (In making the ruling, the judge acknowledged a “Catch-22,” saying that the government claimed “as perfectly lawful certain actions that seem on their face incompatible with our Constitution and laws while keeping the reasons for their conclusion a secret.”)

The U.S. has also sought to dismiss a lawsuit brought by family members over Awlaki’s death and that of his 16-year-old son – also a U.S. citizen -- who was killed in a drone strike.

When does the drone war end?

The administration has reportedly discussed scaling back the drone war, but by other accounts, it is formalizing the targeted killing program for the long haul. The U.S. estimates there Al Qaeda in the Arabian Peninsula has a “few thousand” members; but officials have also said the U.S. cannot “capture or kill every last terrorist who claims an affiliation with al Qaeda.”

Jeh Johnson, who just stepped down as general counsel for the Pentagon, gave a speech last month entitled, “The Conflict Against Al Qaeda and its Affiliates: How Will It End?” He didn’t give a date.

John Brennan has reportedly said the CIA should return to its focus on intelligence-gathering. But Brennan’s key role in running the drone war from the White House has led to debate about how much he would actually curtail the agency’s involvement if he is confirmed as CIA chief.

What about backlash abroad?

There appears to be plenty of it. Drone strikes are deeply unpopular in the countries where they occur, sparking frequent protests. Despite that, Brennan said last August that the U.S. saw,“little evidence that these actions are generating widespread anti-American sentiment or recruits.”

General Stanley McChrystal, who led the military in Afghanistan, recently contradicted that, saying, “The resentment created by American use of unmanned strikes ... is much greater than the average American appreciates. They are hated on a visceral level, even by people who've never seen one or seen the effects of one.” The New York Times recently reported that Pakistani militants have carried out a campaign of brutal reprisals against locals, accusing them of spying for the U.S.

As for international governments: Top U.S. allies have mostly kept silent. A 2010 U.N. report raised concerns about the precedent of a covert, boundary-less war. The President of Yemen, Abdu Hadi, supports the U.S. campaign, while Pakistan maintains an uneasy combination of public protest and apparent acquiescence.

Who to Follow

For reporting and commentary on the drone war on Twitter:

@drones collects op-eds and news on well, drones. (Run by members of the Electronic Frontier Foundation, which has been outspoken about privacy concerns in the use of domestic drones, but it also covers national security.)

@natlsecuritycnn has breaking news.

@Dangerroom from Wired covers national security and technology, including a lot on drones.

@lawfareblog covers the drone war’s legal dimensions.

@gregorydjohnsen is an expert on Yemen, who is closely following the war there.

@AfPakChannel from the New America Foundation and Foreign Policy tweets news and commentary on Afghanistan and Pakistan.

Correction: An earlier version of this piece referenced a speech given by former State Department legal adviser Harold Koh. The speech was in fact given by Jeh Johnson, then general counsel for the Pentagon.