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.