Minecraft wiki entry for The_Update_that_Changed_the_World, associated text: Each biome now has a "temperature" value. Biomes with similar temperature values are more likely to generate next to each other.
Biomes can be divided roughly into 4 categories based on how their temperature affects precipitation:
In hot biomes, it never rains.
In warm biomes, it rains at all altitudes.
In cool biomes, it rains at low altitudes and snows at high altitudes.
In snowy biomes, it snows at all altitudes.
Many existing biomes have been updated, and some have received slightly different variants where some key aspect of the generation is different: