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 Minetest had version numbers, biomes were undefinable and only consisted of 4 materials: grass, stone, light and "water" (as seen on the right.) When 0.0.1 came out it brought actual biomes to the table, but before 0.2.20110529 Pre-Alpha map generation was a bit of a mess. Biomes would cross into each other, and were generally undefinable. So far 0.4 Dev has the most stable biome generation.
For more detail on the evolution of the map generator see the Map Generator Evolution page.
Forests are the most common biomes in minetest. Trees and apple trees spawn here naturally. Generally forests can spawn lakes, ponds, and rivers inside of it. Forests usually surround plains.
Introduced in 0.4.dev-2012040 dev snapshot, deserts contain large amounts of desert sand and desert stone. Cacti and dry shrubs also spawn here naturally in clusters on desert sand. Being the latest biome added in to minetest, these are still a bit buggy. Deserts spawn underground rivers and lakes which can be over 200 blocks deep.
Jungles contains large trees which grow thickly packed together. Jungle Grass and Jungle Trees bearing Jungle Leaves spawn here naturally. These are not generated naturally anymore in the latest 0.4 dev version. Jungles were added back in with 0.4.5-dev of march 16,2013. As of 0.4.7, jungles are not activated by default. “v6_jungles” has to be added to the variable
mg_flags into the minetest.conf to cause the map generator to generate jungles.
Mountains 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.
Plains are a quite flat biome. Large amounts of Dirt nodes and Grass Blocks, on which flowers and Grass may appear naturally in this area (since 0.4.7), are in this biome. Before 0.4.3, long lines used to be carved in plains.
Oceans are 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. Oceans are commonly referred to as lakes, but are called oceans because they can be so huge, sometimes even 200-350 blocks across.
Not classifiable as a biome, but still frequent enough to be classified as a technical biome, islands occur many times in oceans. These could even be referred to as a sub-biome, being a product of another biome. Islands usually raise out of the ground 3-10 blocks, but can get as large as 40-80 blocks tall.
A giant structure that seems to only happen in 0.4, plateaus are very common around mountain biomes. Plateaus can even float above ground and generate miniature biomes on top of it.
Water caves are caves that only occur at sea level, which turn into a huge underground river or lake. These caves can lead thousands of blocks down and are extremely easily to get lost in.
Fairly common below -95Y, these lakes are caverns flooded with lava. Ores are extremely common around these lakes (see screenshot). However, often these caverns are not linked to, and it is very difficult to reach them.
Dungeons are hollow underground structures made of Cobblestone and Mossy Cobblestone. Many pathways and cobblestone Stairs connect a complex system of rooms.
In 0.4.7, dungeons are not activated by default. “dungeons” has to be added to the variable
mg_flags into the minetest.conf to cause the map generator to generate dungeons.