Nether Fortress

Nether Fortresses are structures that are naturally generated in the Nether. They are bridge structures constructed of Nether Bricks, Nether Brick Fences, and Nether Brick Stairs. They are the only places where Nether Wart, Blaze spawners and Wither Skeletons can be found.

Structure
Nether Fortresses are very large complexes made mainly of Nether Bricks. Nether Fortresses can tunnel through Netherrack, and the inside of the tunnels will be cleared of Netherrack by the terrain generator. However, at broken bridge sections, the terrain is not cleared, which may create a tunnel that leads straight into a wall of Netherrack. Their main structure takes a shape of bridges with supporting pillars outside and corridors inside, both which have 3-blocks wide walking space. The bridges can be broken at the end. The interior of the structures have 1x2 windows with Nether Brick Fences as the windowpanes. The fences also form gate-like structures at the entrances of some rooms and corridors. The rooms include:
 * Lava well rooms
 * Rooms with Nether Brick Stairs in the center and Soul Sand at sides of the stairway containing 2 2x5 rows of Nether Wart growing on it.
 * Rooms with Blaze spawners (normally 2 per fortress), which can have Nether Brick Stairs leading up to them. The rooms also have Nether Brick Fences.
 * Rooms with four exits. Some of them lead only to Netherrack.
 * Rooms with a "stairway" of Nether Brick placed against a wall, leading up to the floor above.

In a nether fortress, there are two sections: an indoor section and an outdoor section. The indoor section has a netherbrick roof and is where chests are spawned. The outdoor section has no netherbrick roof, although it can tunnel through netherrack, making it seem like it has a roof. The outdoor section is where blaze spawning rooms are found. In every fortress, there is one lava well room, which is the single connection between the indoor and outdoor sections.

Nether Fortress Loot
As of Minecraft 1.6, Nether Fortresses generate Nether Fortress loot with chests in the indoor sections placed at some interior points where corridors turn at right angles. These chests can contain any of the following:

Spawning
Nether Fortresses may be spawned in inconvenient places, where half of or even the majority of the fortress is buried in Netherrack. The interior will not be filled, however; all hallways and passages will be clear except for open walkways and bridges. It is rare, but possible for Glowstone to spawn inside of the fortress pathways.

Mobs
Nether fortresses spawn Magma cubes at a much higher rate. They also spawn two exclusive mobs which cannot be found anywhere else, Blazes and Wither skeletons.

Blazes are desired because of their drop - blaze rods, as well as the high amount of experience they give. Wither Skeletons are mostly desired for their Wither Skeleton Skulls, which are necessary to create the Wither. Ultimately those who spawn the Wither are aiming to acquire the nether star which is used to construct the beacon.

The spawning algorithm checks if a chunk has a fortress within it. The algorithm then checks for the "bounding box" of the original fortress. It then allows mobs to spawn up to 7 blocks above the walkway, if there is a block beneath them. The entire width of Nether fortresses are taken into account, including junctions and special areas. Fortresses generated before 1.4 lost their bounding box when the fortress was unloaded for the first time due to a bug, and will not naturally spawn wither skeletons, skeletons, and blazes (apart from ones from spawners) after loading them again, even in newer versions that have fixed the bug

Any additions to fortresses will not spawn these rare mobs. Spawning is not affected by replacing the Nether brick with other materials.

Trivia

 * Often, Nether Fortresses spawn with no Nether Wart gardens in them at all.
 * The interiors of Nether Fortresses are safe from Ghasts, because they are enclosed and are made out of materials that Ghast fireballs can not destroy. However, the bridges are open to the sky, as are many platforms.
 * Nether Fortresses use a separate list of spawnable mobs from the rest of the Nether. This allows Blazes to spawn naturally and makes Magma Cubes much more common. Ghasts are not allowed to spawn, preventing them from being trapped inside.
 * Nether Fortresses will still be generated even if the "Generated Structures" option is turned off.
 * Mushrooms can spawn in Nether Fortresses.
 * Skeletons can spawn in Nether Fortresses. When the game tries to spawn a Wither Skeleton it has a 20% chance of spawning as a skeleton instead.
 * Glowstone can spawn in the corridors of Nether Fortresses.
 * Nether Fortresses which spawn high above the lava oceans of the Nether can create huge walls of Nether Brick.
 * Nether Fortresses tend to spawn in "strips" aligning along the Z axis (north/south) in the nether. (Tested over 200 seeds so far, the trend is strong in all tested seeds - confirmed with AMIDST.)
 * If you don't see a Nether Fortress within 100 blocks or so north/south of your starting position, it's best to search diagonally or east/west. If you continue searching north/south, you may be between strips and never find one.
 * On the flip side, if you've found a Nether Fortress, it's best to search north/south from there to find more.
 * When trying to spot nether fortresses, a potion of nightvision can help considerably.
 * Sometimes, two or more Nether Fortresses can spawn very close or even within each other, ultimately creating an even larger cumulative Nether Fortress.
 * If you are trapped in the Nether with your portal unlit, try finding a Nether Fortress' chests. Sometimes, a Flint and Steel may spawn in one.

Gallery
Nether-Festung Fortaleza del Nether Ruines du Nether ネザー砦 Nether Fort Адские руины 下界要塞