Minecraft wiki entry for Development_resources, associated text: Java Edition Classic level format used by Classic.
Java Edition Indev level format used by Indev.
Java Edition Alpha level format used by Alpha (previously Infdev).
Java Edition level format used by 1.0
Anvil file format used by 1.0 (from 1.2)
Region file format used since Beta (from Beta 1.3).
Chunk format for the individual 16x256x16 chunks contained in regions.
Map item format used to store the colors on the craftable map items.
player.dat format used to store the state of players.
Structure block file format use to store structures saved by the structure block.
villages.dat format used to store the state of villages in a level.