Reading an old XKCD comic I found this sentence (you must hover the mouse over the picture to see it):
There are probably children out there holding down spacebar to stay warm in the winter!
I've never heard "in the winter", only "in winter". I've found a discussion on WordReference, where they seem to say that there is not much difference and it comes down to personal taste, but they don't sound like they are sure. The user that asked the question suggests that this might be a difference between American English and British English, but at that point no one answered.
So, what's the difference between "in winter" and "in the winter"?