Before adjourning for Christmas, the Senate passed a "tax extenders" package that the House already passed a couple weeks ago. It's expected to get signed by President Barack Obama in short order.

The short version is that a bunch of tax breaks — costing about $42 billion a year, and mostly helping corporations — expired at the end of 2013, and Congress just retroactively renewed them so that they expire at the end of 2014 and apply for this year's taxes.

The long version is … well, here we go.

What tax breaks expired at the end of 2013?

Fifty-five tax breaks expired at the end of last year. A full PDF list can be found at the Joint Committee on Taxation (JCT). Extending those breaks for another year would cost $41.6 billion.

Tax credits and deductions for businesses account for most of that price tag — about $23 billion, plus some corporate energy breaks. The biggest single corporate tax extender is probably the Research and Experimentation (R&E) tax credit, which lets corporations and business owners deduct a certain percentage of their research and development spending. That's been in the tax code since the Reagan tax cuts of 1981, but has been renewed 15 times since then, never permanently. Other business breaks include:

The catchily named Subpart F Exception for Active Financing Income, which lets corporations defer taxes on certain kinds of financial profits generated in other countries, including offshore tax havens.

"Look-through treatment," which makes it easier to move profits between foreign subsidiaries tax-free.

"Bonus depreciation" and Section 179 deductions, which let companies deduct more of the cost of equipment when it's purchased rather than over the whole period it's used.

A provision letting restaurants and other retail businesses deduct the cost of improving their building over 15 years, instead of the usual 39.

The Work Opportunity Tax Credit, which rewards employers who hire unemployed veterans, recipients of food stamps or welfare, or ex-felons, among other vulnerable groups.

The New Markets Tax Credit, which lets companies and investors write off contributions to organizations providing loans and investments in low-income communities.

Another set of business breaks focus on energy issues:

The Renewable Energy Production Tax Credit, the second biggest tax extender after the Research & Experimentation credit, which lets renewable energy providers reduce their taxes by a cent or two per kilowatt hour of energy produced through renewable means. The exact amount of the subsidy varies between wind, geothermal, hydroelectric, and other energy sources.

The Biodiesel Production and Blending Tax Credit, which gives producers and blenders of biodiesel fuels $1 per gallon produced.

There are also a handful of individual provisions among the extenders:

Why were they all expiring? Why hasn't Congress just enacted them permanently?

These credits were passed as temporary measures, most often as a method of short-run economic stimulus. Then people and corporations grew accustomed to them, and they were renewed often enough that for them to lapse entirely would throw a serious wrench into financial planning — at least for rich individuals and corporations.

Politically, their permanently temporary status has a lot to do with the higher cost of enacting them permanently. Recall that a one-year extension will cost about $41.6 billion. A permanent extension would cost $700 billion or so over 10 years, which is the period the JCT scores things over. That's both tougher to pass because of the raw price tag and because deficit worries are likelier to make politicians demand that the extension be paid for.

This year's extension isn't being paid for at all. But when Congress looked set to permanently enact a number of tax extenders without paying for them, the White House threatened a veto, partly because it also wanted some low-income tax credits extended but also because it wanted a plan to offset the costs. Working that out is tricky. Democrats are likely to demand the closure of other corporate tax breaks that Republicans want to keep, while Republicans are likely to demand spending cuts that Democrats find unacceptable.

What does the legislation that passed do?

The passed tax extender bill extends almost every provision with a few notable exceptions. One of the biggest exceptions was the Health Coverage Tax Credit, which pays health premiums for certain pensioners who aren't yet eligible for Medicare, and who either have had their pensions taken over by the Pension Benefit Guaranty Corporation because their employers went bankrupt or lost their jobs due to trade competition. Retirees from the Delphi auto parts company were among the biggest beneficiaries of the credit. Many live in Ohio, leading its two senators, Sherrod Brown (D-OH) and Rob Portman (R-OH), to vote against the tax extenders package.

Additionally, the package includes the ABLE Act, which lets people with disabilities and their families save up to $100,000 tax-free for disability-related expenses. That's particularly important because programs that people with disabilities rely upon, like Medicaid, often impose asset limits that make saving for other expenses impossible. The ABLE Act offers a way around that.

What's the case for extending these?

There's broad consensus in DC that at least some of the tax extenders are good policy. The Research and Experimentation Credit is very popular, for instance, and there's a good deal of research suggesting it does spur more research spending — at least the kind of spending it counts toward. A number of the individual breaks, like the teaching expenses deduction or the IRA charitable donation provision, are very popular as well.

Businesses have also been counting on the provisions being extended, which led some to argue for passage on business certainty grounds. Sen. Chris Murphy (D-CT), who voted yes, argued that the one-year extension "falls far short of ideal, but it provides certainty for the thousands of families and businesses in Connecticut and across the country that are counting on these provisions."

What's the case against extending these?

Many people have substantive problems with one or more of the provisions in the package. Traditional energy companies hate the renewable energy subsidy, for instance, and liberal tax wonks loathe some of the corporate tax provisions. Citizens for Tax Justice notes that the Congressional Research Service has judged "bonus depreciation" to be an inefficient way of boosting the economy, argues that the definition of "research" in the Research and Experimentation credit is so vague as to include things like "redesigning food packaging," and claims the active financing exception "provides a tax advantage for expanding operations abroad."

More generally, the current approach of annually renewing these provisions is a waste of congressional time, one which provides more avenues for lobbyist influence on the tax code and reduces time available to pass spending bills and other, more necessary legislation. "Not all tax extenders must go, but the idea of tax extenders must go," Alan Cole of the right-leaning Tax Foundation argues. "All of them should be made permanent or allowed to expire."

Others, like Senate Finance Committee chair Ron Wyden (D-OR), argue that since the extenders will only be law for two weeks before expiring again, they lack any incentive effect. "This legislation wants to do the impossible: to incentivize behavior retroactively," he said in a statement. "It cannot change anything taxpayers did six, eight, or 10 months ago. Those decisions have already been made, and those actions have already been taken."