Minecraft wiki entry for Nether_Fortress, associated text: Straight bridges.
Up to 5 plain four-way crossings.
Up to 4 four-way crossings with arches made of nether brick and nether brick fence.
Up to 4 small rooms with a single entrance, and full-block "stairs" leading to the roof, which may have a single path leading out.
Up to 2 blaze spawner platforms: structures consisting of three full-block "stairs" leading to a small platform fenced with nether brick fence, with a blaze spawner in the center.