Stronghold

Strongholds (previously unofficially referred to as ruins) are part of the 1.8 "Adventure" update. They are a new type of structure that occurs naturally underground, and they contain multiple rooms, corridors, stairs, doors and other aspects, as of the version of 1.8. Stone Brick, Mossy Stone Brick, Cracked Stone Brick, and Iron Bars were revealed in the same image as being the materials that make up the strongholds. Unlike other 1.8 structures, only three Strongholds spawn per world. Also found in strongholds are Doors (wood and iron), Bookcases, and Buttons. Some treasures found in strongholds include Paper, Books, Coal, Gold Ingots, Iron Ingots, Compasses, and Red Apples.

Stronghold
The name "stronghold" for this feature has been confirmed by Notch in a Google+ post.

It is possible that boss monsters will be added to the strongholds. Jeb said in a Twitter post that he plans on adding this feature but that it is not done yet. Strongholds can be big depending on the map seed.

1.8 Beta
Strongholds appear in the Beta 1.8 version. They can appear in worlds created prior to 1.8, provided the player has not yet visited the areas where a Stronghold spawns. While the world map is effectively infinite, the number of strongholds is limited to two-three per map,.

Stronghold appearance
Strongholds vary in size (though they can be very large). The Strongholds are Labyrinth-like and contain several doors and rooms. The strongholds are mostly made of Stone Brick, Mossy Stone Brick, and Cracked Stone Brick. Doors of both types with buttons and Iron Bars are also located here with stone slabs forming stairs and central pillars of some rooms. Torches are scattered around this place. Some rooms contain Chests with various items.

Stronghold features various types of main rooms, such as Prison Cells, Fountains, Libraries, Storage Rooms, 4-way Intersections, Bridge stairs, interconnected with various corridors, staircases, and spiral stairs.

One can also find rooms containing Bookshelves, Fences, and Wooden Planks arranged in what would appear to be a library which also contains Cobwebs. Strongholds have also been known to contain fountains. All treasures mentioned before are accounted for in the pre-release. The Map Generator frequently generates strongholds underground, often unexposed to the surface and only visible within caves, possibly even underwater caves. The Map Generator is apparently incapable of discriminating between strongholds and standard blocks, as map generation features such as Underground Ravines and Abandoned Mine Shafts cut indiscriminately through strongholds, often making the strongholds difficult to navigate. It is unclear whether this functionality is intended or not. Insofar as the 1.8 Beta, the strongholds are vacant except for regular mobs that spawn under normal conditions. Strongholds are the only location where Silverfish can spawn, created when some of the stones are mined. No new NPCs have been added.

Chests can be found on the upper floor of library rooms and on the upper floor of 4-way intersections. Chests in libraries contain paper and books while the intersection chests contain coal, bread and red apples.

Coordinates
All Strongholds are located at random in a circle between 640 and 1152 blocks from the origin, 0/0. Due to the generation algorithm, the Stronghold proper may extend further in or out of that circle.

You can find a stronghold at coordinates/seed:

Seed: 1448524118

X: -873

Y: 35

Z: 44

You can also find an NPC village in this map at:

X: 74

Y: 65

Z: 375

or another one at:

X: -178

Y: 72

Z: 1113

Abandoned mine shaft and dungeon at:

X: -876

Y: 22

Z: -12

There is a ravine at

X:-274

Y:64

Z:231

Surface zombie dungeon at

X: -214

Y: 62

Z: -56