Some chicken breeds lay year round (save when they go broody). One of our employees who's at a mid-latitude in the US reports that any girls who stop laying during the winter begin again regularly--and surprisingly precisely--on February 1 or 2, about halfway between the winter solstice and the spring equinox.
If you live in an area where day length doesn't change much, your chickens may begin laying sooner (or never stop). If you live in an area where winter days are very short and cold, they may begin laying again later in the year. While it's the increasing light that triggers increased laying, if they're using lots of calories just to stay warm, they may not be "going full speed" until it warms up a little. When they begin laying again will also depend on the age of the bird, as older hens tend to lay more seasonally, and on their breed. Some show breeds, like the white frizzle Polish, above, lay very few eggs in general, and even fewer in the winter.
There are a number of reasons your chickens might not be laying, though, other than time of year... so if you suspect there's something more to it than that, have a look at this list of other possible reasons your chickens aren't laying.