Villages
A good place for loot, shelter, and trades, villages are one of the most useful structures in the game.
Abandoned Mineshaft
Mineshafts look like they will have a lot of loot, but actually suck. It is usually not worth the danger to go in the m , but if you have good gear and want a challenge, this is a good place for you to explore.
Above: a cave spider spawning area.
Dungeon
Dungeons are underground structures that contain a spider, zombie, or skeleton spawner and two chests. You can deactivate the spawner by doing what is shown below. (works for all spawners)
Igloo
Igloos are small structures spawning in snowy biomes, and on the top floor, there is a furnace, a crafting table, a redstone torch, and a bed, making it a decent base if you light it up and add a door. There is also white carpet on the floor, and 50% of igloos have a ladder leading to the secret igloo basement under the carpet
Stronghold
Strongholds are large underground structures that contain several rooms, including one portal room, usually 1 or 2 libraries, sometimes a well room, many cobblestone staircases, spiral staircases, jails, and chest halls. The end portal room contains a silverfish spawner, lava, and the portal itself, which can have any amount of eyes of ender already in the frames.
Tip: Before you go to the end, you should set up a little "minibase" in the portal room by sealing off the entrance (probably with a door) destroying the spawner, placing a double chest full of backup items in case you die, and lastly, a bed to sleep in to reset your spawnpoint.
Nether Fortress
Nether Fortresses are large structures found in the nether containing many hallways, bridges, and a few blaze spawner rooms. It can be hard to get into a fortress because they often spawn in the air, and players sometimes miss them because they are roughly the same color as netherack.
Mansion
This huge structure is very dangerous and can be fatal to even the best players, but can also have good loot. They contain normal mobs (zombies, skeletons, Etc.)and also have vindicators and evokers. They can also contain many different types of rooms, including jails, storage rooms(weirdly no loot), and also some secret rooms.
End city
These are large structures spawning on the end islands, and they contain shulkers, and can also have an end ship, which contains an elytra. End cities also have varied loot, including iron and diamond armor (often have enchantments), weapons, and more!