The winter season has fewer units on the market, and sellers tend to need to move from their property. You can use that to your advantage to get a favorable deal.
Winter has fewer buyers in the market. Looking for a home in the winter can be inconvenient, and people are less likely to move. Families also tend to be on a September to June cycle because they are unwilling to move their children to a new town in the middle of the school year. Fewer buyers means less competition.
Lenders also usually have fewer loans to process and less paperwork to deal with (though this can change quickly if rates fluctuate). With lenders less hassled, you can expect a smoother process to get approved for a mortgage. But, as reported in Bankrate.com, there are exceptions to this rule, most notably in warmer parts of the country (especially Florida), ski towns, and in parts of the country where demand is so strong that it will not slacken during the winter months.
Finally, as all savvy shoppers know, after the holiday season comes the season of bargain opportunities. This includes houses, as well.