Biome



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 that different areas with varying heights, temperatures, humidity ratings and foliage colors are created.

Before the Halloween Update, when a map was generated it had either a Snowy or Grassy theme. But after the update, a single world includes all themes in a logical fashion determined from the biomes graph.

A demonstration can be viewed here.

Biome Types
Biome types may be easily distinguished by the differentiating grass and leaf colors in conjunction with the kind of blocks present, like cacti in deserts and pine trees in pine forests. They are randomly created during the generation of the world and are affected by the starting seed.

There are nine biomes in Minecraft 1.0.

History
Biomes were added in Alpha 1.2.0, also known as the Halloween Update. In Beta 1.8, biomes got an overhaul, removing some biomes and others replaced with nine fractal-based biomes - some old, some new.

Difficulty
It can be difficult to play and gather sufficient resources if one starts in the middle of a biome with no trees, such as the desert, savanna, tundra, and especially the ocean.

There have been multiple reports of players spawning on these tiny islands in the middle of a vast ocean with no trees for several Minecraft days in any direction. It appears that these desert islands generate in the Ocean biome, where animals cannot spawn (on the water); sometimes the island spawns with no trees so players have to delete the world and start again, swim great distances to a landmass, or search for nearby underwater openings into abandoned mineshafts to acquire wood. This is not quite as bad of a problem if one starts the map in Creative Mode since the player is able to fly, make a boat, or plant trees. It is still undetermined whether this feature is a bug or not (Notch didn't say it was a bug, but the community seems to think it is).