"To the right of the player is a Taiga (mostly over the ocean), to the left is either a Forest, or Woods, I can’t remember. In the distance is probably tundra." -Notch
According to Wikipedia, Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems.
In Minecraft, from the Halloween Update onwards, this means different areas, with different heights, temperatures, humidity, and foliage colors.
Before the Halloween update, when a map was generated it got a 'theme', i.e. Snowy or Grassy. After this update, a single world will include all themes in a logical fashion.
Demonstration video
Current Savegames
Worlds on current savegames will not be changed. However, any new chunks generated (by the player moving outside of the current world's bounds) will use biomes, just as The Nether will be generated when you first enter it.
Biome Types
There are 11 Biome types: (Note: these names come from Notch's notes, The Slip was renamed to The Nether)
Biomes Graph
- Rain Forest
- Swamp
- Seasonal Forest
- Forest
- Savanna
- Woods
- Taiga
- Desert
- Grassy Desert
- Tundra
- The Nether
- (This isn't an official biome, as it is not found in the normal world. But it does have unique geographical features, making it similar to a biome.)
Remaining Work
Remaining work on biomes include variable elevations in certain biomes (flat elevation in desert biomes etc.) in addition to improving the border between arctic and tropical biomes.