Dirt

Dirt is a basic block found abundantly in the Overworld. It is found at any altitude, and comprises the majority of the upper terrain layers in most overworld biomes. Dirt can be thought of as a filler block, used during world generation to fill gaps between resources and features and provide shaping material for terrain, but it can also be collected and utilized for building. Grass, snow, and mycelium can spread/collect over the top layer of Dirt blocks; mining these will drop ordinary Dirt blocks. Podzol cannot spread to dirt blocks, because it is a variant of the dirt block itself. There are approximately 1850 dirt + grass blocks per chunks in Plains, Forest, Tundra, Jungle, and Extreme Hills biomes. There is significantly more (as much as 3000) in chunks with high mountains.

When a dirt block is adjacent to a grass block and is exposed to a light level of at least 4 (e.g. from torches or sunlight), it will eventually be converted into a grass block at random intervals (see grass for growth details). Mycelium spreads in similar fashion, but requires a light level of at least 9.

Coarse Dirt is a variation of dirt in Mega Taiga, Mesa and Savanna biomes. The only difference to regular dirt is that grass cannot spread to it. Hoes can be used to convert coarse dirt into regular dirt.

Obtaining
Dirt will always drop a resource block, regardless of how it was mined. It may be mined by hand fairly quickly, but using a shovel will significantly speed up the process. Dirt has very low blast resistance, so any form of a explosion will clear a sizable area of it. But using explosions isn't a good idea since most of the dropped dirt will be destroyed by the explosion.

Crafting
Since this recipe produces four blocks whereas only two dirt blocks were needed, and the fact you can till coarse dirt back into the regular stuff, this can be a simple, if rather inefficient, way of converting spare gravel into dirt.

Usage
Dirt is generally considered an obstacle during mining, though it is nevertheless useful in Survival mode for quick temporary structures like walk ways, quick easy houses and pillar jumping; it is easily mined even without tools, large quantities are generally readily accessible, and hostile mobs can not generally attack through it. A player caught outside when night approaches, for example, can remain fairly safe by sealing themselves into a small dirt cavity, or pillar-jumping up 16 or 20 blocks.

Large dirt structures can even serve as shelter for extended periods, if lit sufficiently to prevent mob spawns. Dirt structures can be outfitted with Doors and any other creature comforts, the same as most other blocks. Players will usually replace the dirt with stronger blocks over time (if only for increased blast resistance) but the dirt serves to block mob lines-of-sight, and to lay out the arrangement of the building.

Dirt is also important for its ability to grow a variety of items. Saplings, sugar canes, and mushrooms can be planted directly in dirt (under appropriate conditions for each). Using a hoe on placed Dirt blocks will turn them into farmland blocks, enabling wheat seeds, pumpkin seeds, melon seeds, potatoes, and carrots to be planted on it. Farmland will revert to dirt unless something is planted on it or it is irrigated by nearby water.