(CNN) The US Justice Department has appointed a special counsel to lead the federal investigation into Russian interference in the 2016 election, including potential collusion between President Donald Trump's campaign associates and Russian officials.

Here's what you need to know.

What is a special counsel?

A special counsel is a lawyer appointed to lead an independent investigation and, if necessary, to prosecute anyone suspected of crimes.

A special counsel is typically appointed when the usual investigative bodies under the Justice Department, such as the FBI, have a conflict of interest in carrying out a probe.

The law states more broadly that the Attorney General (or acting Attorney General, as in this case) can appoint a special counsel under "extraordinary circumstances" or when in it is in the public interest to do so.

The special counsel must come from outside the government, the idea being that he or she should have the greatest possible level of impartiality and be removed from the usual chain of command.

The law says that the counsel should be a lawyer with a reputation for integrity and impartial decision-making,

The special counsel answers to the Attorney General. But in this case Mueller will answer to Rosenstein, as Attorney General Jeff Sessions has already recused himself from any Russia-related investigations.

Sessions recused himself after it emerged that he had failed at his Senate confirmation hearing to disclose two pre-election meetings with Russia's ambassador to Washington.

Why now?

Mueller's appointment aims to quell the wave of criticism that Trump and his administration have faced since Trump fired FBI Director James Comey last week in the middle of the FBI's intensifying investigation into contacts between the Trump campaign and Russian officials.

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Deputy Attorney General Rosenstein -- who has not publicly commented on why he or the Justice Department made the decision to appoint Mueller -- has been drawn into controversy surrounding the Russian investigation.

Administration officials repeatedly said that the decision to fire Comey was based on Rosenstein's advice, which was detailed in a memo -- but Rosenstein has said he is unhappy that the White House tried to pin Comey's removal on him.

Opposition figures from the Democratic Party have long called for a special counsel. But so have several members of Trump's own Republican Party, like Senator John McCain, as the issue has dominated Trump's first months in office and has made it impossible for the Republicans to focus on their domestic agenda.

Can Mueller prosecute Trump?

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If Mueller finds incriminating evidence against anyone, he can seek indictments. But it is less clear if he can do this in Trump's case -- presidents hold a certain amount of legal immunity. Mueller could choose to seek the impeachment of President Trump, but that would require Republicans in Congress to turn on their leader.

Two presidents have been impeached, but neither Bill Clinton in the 1990s nor Andrew Johnson in the 1860s was convicted. Richard Nixon resigned before he could be impeached.

Can Mueller be fired?

Yes, but only by the Attorney General, or acting Attorney General. So unlike in "special prosecutor" cases in the past, Trump cannot fire Mueller.

Is a special counsel and special prosecutor the same thing?

Almost, but there are some key differences. When people were calling for a special prosecutor in this case, they used the term interchangeably with special counsel.

But the former is an expired term. In the past, a special prosecutor was typically appointed by the president, but following the Watergate scandal that brought down President Richard Nixon, the rules changed to remove this direct power from the president.

After several changes in the law, the role of special prosecutor was eventually scrapped, and a new law in 1999 created the role of special counsel. This role can only be appointed by an Attorney General, or an acting Attorney General.

A special counsel must also be a lawyer, where in the past, some special prosecutors had other professional backgrounds.

What do we know about Robert Mueller?

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Mueller, 72, is widely seen as a non-political and meticulous investigator who is respected on both sides of the political aisle in Washington. He was nominated as FBI director under Republican President George W. Bush but stayed on for most of Democratic President Barack Obama's term.

He was Comey's predecessor and served as the second-longest FBI director in history, only behind iconic and controversial director J. Edgar Hoover.

Mueller saw the bureau through a time of tremendous change following the 9/11 attacks. His tenure saw the rise of al Qaeda as a threat, the vast expansion of US surveillance capabilities and an increase in the threat from home-grown terrorism.

Mueller is also no stranger to high-profile cases. In 2015, he conducted an inquiry into the NFL's handling of a video of former running back Ray Rice punching his then-fiancée. And last year, Mueller was asked by the consulting firm Booz Allen Hamilton to conduct a security review after a contractor was arrested for allegedly stealing national security secrets.

How many Russia investigations are there now?

Three -- or five, depending on how you break it down.

In Congress, both the House and Senate are conducting investigations, and the Justice Department is holding its own.

In Rosenstein's letter, he made clear that Mueller was taking on the Justice Department's investigation that had been until now led by the FBI. So that's three.

But the House and Senate both have two committees each working on the probe. They include a House Intelligence Committee, the House Oversight Committee, the Senate Intelligence Committee and the Senate Judiciary Committee Subcommittee on Crime and Terrorism. So with the Justice Department, that makes five.

Mueller's appointment essentially strips power from the FBI from leading the federal investigation, although FBI members could continue to play a major role in the probe. Mueller will have wider scope than the FBI, however, and can openly investigate members of the White House.