Stronghold

Strongholds (sometimes unofficially referred to as ruins) are a possible addition to the 1.8 "Adventure" update. They are a new type of structure that seems to occur naturally, and appear to contain multiple rooms, doors and other aspects. 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. Strongholds spawn just once per world. Also found in strongholds are Doors (wood and iron), Bookcases, and Buttons. Some treasures found in strongholds include Paper, Books, Coal, Iron Ingots, and Red Apples.

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 Pre-release
Strongholds appear in the leaked Beta 1.8 pre-release. Strongholds apparently only generate once per world and 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 with Doors of both types, Iron Bars, and Chests. One can also find rooms containing Bookshelves, Fences, and Wooden Planks arranged in what would appear to be a library. 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 Pre-Release, the strongholds are vacant except for regular mobs that spawn under normal conditions. A new NPC was added, the Silver Fish. More NPCs may be added before the official 1.8 release.

As nothing is set in stone, all of this information should be taken with a grain of salt.