Minecraft wiki entry for Development_versions, associated text: Added many new villager trades, for each villager profession.
Villager trading prices now also depend on demand and player's popularity in the village.
Villagers now resupply their trades up to two times a day, if they get to work at a job site block.
Updated the villager trading UI.
Villagers now level up from gaining experience by trading with a player.
Villagers have 5 levels: novice, apprentice, journeyman, expert, and master.
A little experience bar is shown at the top of the trading UI. Once the bar fills up from trading, exiting the UI and waiting briefly will level up the villager and show pink particles similar to those shown when refreshing trades in previous versions of the game.
Experience that exceeds the level up amount seems to roll over between level ups.
The experience bar disappears once the villager becomes master level.
Every time a new level is reached, 1-2 new trade offers appear.
The villager's experience gained per trade is based on the trade itself, the level the trade was unlocked at (novice, apprentice, etc), and the current level of the villager.
Lower level trades will give less experience towards a level up once the villager is a higher level.