I get this question quite a bit, so I thought I'd give a quick answer. Many insects die in the late fall, leaving eggs buried in the ground or encased in some protective covering to perpetuate the species. Others, like the infamous and devastating pine beetle here in Colorado, continue to live on in the tree it is inhabiting, barely restraining its life cycle. Bees are somewhat like this in that the hive continues on, but with a lot less travel.
Bees make honey as an overwintering food. As the weather turns cooler, the queen nearly halts reproduction as the fall brood will live much longer than their earlier born sisters, who have life spans of about 12 weeks. The autumn-born workers do little travel outside the hive, the biggest hazard to a bee. You can tell how close a worker bee is to death by how tattered her wings are. I ran across a dead bee on the green belt just a few days ago and its wings were a shambles--most likely she died of old age considering how mild the weather has been this December. However, these late season workers are no slouches. They continue many of the internal hive duties that their spring and summer-born sisters engage in.
The girls (all the drones have long been kicked out of the hive to conserve honey and bee bread) still tend to the queen and the brood that will be born in late January and early February. They also meet all the needs of their queen by collecting her feces and depositing outside when its warm--sunny and 45-50 degrees at least. The queen is the only bee that will defecate in the hive--the workers must go on cleansing flights when the weather is warm enough. A healthy worker would rather die than defecate in the hive. This is one of the many reasons a bee hive is the most sanitary of any animal lair in the world. They deposit the dead outside of the hive on those cleansing days.
To survive the eventually brutal temparatures to hit them, the girls cluster around each other and beat their wings to generate warmth. Much like penguins, they answer some kind of call that cycles the bees inward to keep everyone warm. Through this convection of sorts, the area around the bees stays at a balmy 90 degrees or so (oh, lucky them!). In Vermont, where it gets to 40 below all too frequently, this method keeps bees alive and kicking, ready to take advantage of the first days of spring.
Some beekepers chose to tarp or otherwise cover their bees. We are choosing not to since Colorado winters are pretty mild and the extra labor and cost seems kinda pointless. Our experiment is with a solid bottom board compared to a screened bottom board. This expirment is unintentional: when I bought the hive for the Sicilians, our supplier didn't have any screened bottom boards. So we'll see how it works and go from there. Some resources say that you must create additional ventilation if you do a solid board to prevent condensation, however, all these resources come from humid states. That said, when we transistion to top bar hives, we may have to worry about that since the hive is fully enclosed.