From the [Minecraft wiki][1]:

> Nether Fortresses are very large complexes made mainly of Nether
> Bricks. Nether Fortresses can often 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 flat
> 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. Nether Brick Stairs can lead up to
> them. The rooms have Nether Brick Fences. There are normally 2
> spawners per fortress Rooms with four exits. Some of them could lead
> only to Netherrack. Rooms with a "stairway" of Nether Brick placed
> against a wall, leading up to the floor above.**


  [1]: http://www.minecraftwiki.net/wiki/Nether_fortress