Roofed Forests are biomes that were implemented in the Update 1.7.2 along with many others. They are a new type of Forest Biome, including several extra features.
The Trees (and Huge Mushrooms) in this biome are very close together. Most of the forest is dark enough for hostile mobs to spawn. This makes it rather dangerous to live in, but a good mob grinder or mushroom farm. The trees are of the Dark Oak species, with some Birch mixed in.
This biome is also called Black Forest after the forest in Germany.
It also resembles the Darkwood Biome in the Twilight Forest Mod.
It is also one of the few ways to obtain mushrooms in survival, the other ways being swamps, caves, the nether, and mushroom biomes.
It is also the only other way to find a big mushroom.