Minecraft wiki entry for Development_versions, associated text: Textures now use one file per block/item, which are stitched together at runtime – Texture sizes don't need to be consistent.
Any block and item can be animated by putting the frames underneath each other in the right .png – Use an optional .txt to specify frame order and duration.
Special rules apply for water, lava, compass, and clock.
Existing texture packs will "automagically" be converted.
Vanilla textures will be used if texture files are missing.