There are various gererated structures in Minecraft. Evil Minecraft adds various generated structures to Minecraft. Sounds awesome!
These are giant structures located in the desert, they spawn randomly. Inside, there is a mummy spawner, and there are a varitey of mobs, including creepers, zombies, and of course mummies.
Labyrinths are underground mazes filled with dead ends and traps and an indestructible variation of stone bricks. It has been proposed that there is some kind of boss mob in the centre of each labyrinth. It is reccommended that you use lanterns or redstone to let you know where you have been so that you don't get lost in the maze.
Ancient Ruins are basically large, underground dungeons, with spreading moss, vines growing out of the moss, and absolutely loads of mimics and treasure chests. Just watch out for the creepers and whatnot.
These are randomly generated villages, inhabited by the nomadic Pigmen. Pigmen will eventually abandon these villages if there is no player nearby for a few minutes.
A Grand Dungeon is a dungeon with two spawners, double the treasure chests, and maybe a Mimic or two. Sounds fun, huh?
A wizard tower is filled with loot. It may have diamond swords, enchanting rooms, brewing stands, chests, furnaces, and at the top of the tower, pooop
These are probably the most common of the structures. They are areas with graves, and four pillars with pots on the top of them. At daytime, they are normal. At night, zombies spawn. When you destroy the pots of chance, a random surprize it may be a mob, coins, armour and even tools may pop out. These can spawn in any biome, except oceans and what ever biome there in generates the type of grave yard Eg: Snow Biome = a grave yard made of snow bricks.