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. Before 0.2.20110529 Pre-Alpha map generation was a bit of a mess. Biomes were undefinable and often crossed over to
Forest biomes are the most common biomes in minetest. Trees and apple trees spawn here naturally.
Desert biomes contain large amounts of desert sand and desert stone. Cactus also spawns here naturally in clusters. Being the latest biome added in to minetest, these are still a bit buggy. Desert biomes spawn underground rivers and lakes which can be over 200 blocks deep.
Mountain biomes are the most treacherous to scale. Trees and apple trees spawn here naturally. It is quite easy to be killed in a mountain biome if you don't pay attention to where you are going.
The plains biome is the rarest biome. Nothing but grass and caves in this biome. Usually there will be long lines carved into the terrain. Could this be the work of Herobrine?
The ocean biome is an unofficial biome that seemed to be implemented around 0.2.20110731_1 Pre-Alpha. This biome is just a large body of water that can be up to 250 blocks deep. Commonly referred to as "The lake biome", this has the nickname of the "Ocean biome" because it can be so huge.
A giant structure that seems to only happen in 0.4, plateaus are very common around mountain biomes.