An air conditioner is a heat pump that's set up to only move heat from inside to out. A heat pump is set up to push heat in either way so in the winter you can have it push heat into the house. There is a bunch of physics behind it that you can read into.

They get really good efficiency rating because they are just moving energy around by explioting how gases evaporate and condense instead of just converting energy into heat. cold weather does reduce efficiency though.