Dirt is mottled brown in color. If placed and exposed to air on the top, it will eventually acquire an irregular layer of green "grass" that can also become white snow or frost.
Where Found
Dirt makes up the majority of the top layers of solid blocks of a world and may be up to 10 blocks deep. It is very weak and may be mined by hand fairly quickly, but using a spade will significantly speed up the process. If mined by a gold spade, seeds, fruit, and other items will occasionally be spawned as bonuses.
Dirt can also be found in a thin (usually just one block) layer across the top of floating islands.
Dirt, as with other blocks, can be gathered, placed, and walked on. It can have seeds and fruitplanted in it. It also may produce a worm when removed from placement.