Minecraft wiki entry for Development_versions, associated text: Villagers and zombie villagers now have seven skin types which correspond to the following biomes:
Plains
Desert
Savanna
Taiga/mountain
Snowy tundra/snowy taiga
Jungle
Swamp
Villagers now have five profession levels, showing how many trade tiers they've unlocked by a badge of a varying material on their suit.
The first trade tier is represented by a stone badge, the next iron, then gold, emerald and finally diamond.
Some of the professions have less than five tiers in total.
If a villager summoned by a command has a higher level than its natural highest level, its badge can still be seen, however, the player wouldn't be able to interact with it.
Cured zombie villagers will retain their trades when they are converted back into regular villagers.