If you’ve ever applied for a mortgage, chances are good that you’ve heard of Fannie Mae and Freddie Mac; but with some significant recent changes going on, a little review is in order:

These entities were both very good ideas when they were founded, but grew into powerful corporations that were solely profit-driven during the housing market crisis. When the solvency of the two giants was threatened in 2008, the federal government seized them and bailed them out with unlimited taxpayer support. It was never meant to be a permanent solution, but the government regulation has kept the two companies solvent through a tremendous mortgage crisis.

That’s soon to end; a recent Reuter’s report notes that “after December 31, Fannie Mae's bailout funds will be capped at $125 billion and Freddie Mac will have a limit of $149 billion.”

It still sounds like a lot of money—to most people. But there’s concern in the investment community that the Treasury capital will soon run out and Fannie Mae and Freddie Mac will default on bond payments.

The Reuters report quotes the Treasury as saying that the new terms will ensure that “every dollar of earnings that Fannie Mae and Freddie Mac generate will be used to benefit taxpayers for their investment in those firms.”

“We are taking the next step toward responsibly winding down Fannie Mae and Freddie Mac, while continuing to support the necessary process of repair and recovery in the housing market,” Michael Stegman, counselor to the secretary of the Treasury for housing finance policy, said in a statement. Neither Fannie Mae nor FreddieMac will be allowed to retain profits, and both must reduce their massive portfolio holdings at an annual rate of 15%.

What does this mean for you?

The reality is that it may become more difficult to obtain a 30-year fixed-rate mortgage. Locking in the same rate for a long period of time was easy when the investment was insured; it may be a different story in 2013.

A New York Times article noted that “the much more divisive question is whether the government should preserve the benefits that the companies provide to middle-class borrowers, including lower interest rates, lenient terms and the ability to get a mortgage even when banks are not making other kinds of loans.”

The lower rates available because of Fannie Mae and Freddie Mac’s insurance are essentially available because of investors eager to put money into the companies—precisely because of the government guarantee against default.

Would another government entity be able to do the same thing—insure against default and attract the investors necessary to provide favorable terms to borrowers? It’s unclear, even to lawmakers: the same NYT article quoted Representative Barney Frank as saying that “I myself am eager to see whether there needs to be a guarantee.”