Structures

Every specific group of blocks purposefully formed in Minecraft based on coding is part of a natural structure.

The Overworld
The Overworld contains numerous generated structures, at a wide variety of scales.

Small structures
The quantity of most of these features (aside from dungeons, mineral veins, and springs) is biome-dependent; not all features can be found in every biome.

Buildings
Buildings are naturally generating structures that form above ground. They can contain valuable treasure, but traps and puzzles as well.

The Nether
The Nether, though equally vast, contains far fewer types of generated structures than the Overworld.

The End
The End is the final and most barren dimension, with very few types of significant structures, until after defeating the Ender Dragon, which will open gateways to more structures.

Technical details
Structures are generated for a given chunk after the terrain has been formed. The chunk format includes a tag called that indicates whether structures whose point of origin is in that chunk have been generated. If it is false or missing, they will be generated again. Structure generation is based on what is already in the chunk, so (for example) flagging a chunk that has already been populated for repopulation will approximately double the amount of ore in it.

When structures are generated, they can spill over into neighboring chunks that have been previously generated. Thus, a tree at the edge of the generated world (and probably only visible using external tools) may be overwritten by a lake before the player reaches it. It is also theoretically possible for two worlds generated with the same seed, from the same version of Minecraft, to differ slightly depending on the players' travel routes, because the order in which chunks are generated may determine which of two conflicting structures will overwrite or suppress the other.

Trivia

 * In previous versions, before snow cover was solid, a lava lake with floating snow cover above it could be a deadly trap before the snow melted.