Nether Bricks

Nether Bricks are a type of block used to form generated structures in the Nether, making up a majority of Nether Fortresses, forming their walls and supporting pillars. Nether brick does not generate anywhere else in the nether. Currently, the player cannot craft Nether brick. However, Nether brick can be crafted into Nether brick stairs and Nether brick fences. Also, Nether Brick is immune to Ghast fireballs and other attacks making it a suitable shelter material in the Nether. Nether bricks take approximately one second to mine with a stone pickaxe, which is a little shorter than the time required to mine normal stone.

Nether brick does not burn continuously, unlike Netherrack. It appears similar to the brick block, but is textured differently. When walking on it, it makes the same step sound as stone. So far, the only uses for Nether brick are for crafting stairs, fences, and as use as a decoration. Note that wood is a better material for fences because only three blocks of wood are required for six fence posts (three wood = 12 planks = 24 sticks = six fences + an extra two), whereas six Nether bricks are required for the same amount of fence posts.

In the future
Jeb has tweeted a confirmation that in 1.5 (The Redstone Update), Nether bricks will be craftable, though the planned recipe is still unknown. It's also possible that Nether brick slabs will be implemented.