No bees are around in the winter due to the pollen disappearing. This is the opposite to warmer months, when there is plenty of pollen available. Beekeepers actually do very little to keep the bees happy during the winter months. Bees have been around a lot longer than beekeepers, and have developed their own systems for keeping warm.
Honey - Another thing that bees do similar to humans to stay warm is to shiver. The bees on the inside of the cluster shiver their bodies, which generates heat that then radiates outward. The shivering requires a lot of energy, however, and so the bees eat the honey to keep their energy up. This is part of the reason that they cluster around the honey.