Dirt Block

Dirt Blocks are one of the most common block types in Terraria, covering the majority of the world's surface.

Dirt Blocks may be covered by regular grass, Corrupt grass, Crimson grass, or Hallowed grass, which spreads from nearby grass-covered Dirt Blocks. It can also be turned into regular grass by using the Staff of Regrowth.

Dirt occurs naturally almost everywhere throughout the world. All caves above the elevation of 0ft are made of dirt. Below 0ft, in the underground layer, the caves are made of roughly 50% dirt. Small patches of dirt can be found in the cavern layer, though it is mostly made of stone.
 * In all cases, the grass applies to the block as a whole, and will appear on all exposed surfaces.
 * If a grass block is covered by blocks on its four sides, the grass will remain and show as markings on the block. However, if the block is completely surrounded with eight blocks, the grass is lost and the block will revert to regular dirt.
 * Upon world generation, 3 (in a small world), 6 (medium) or 9 (large) Dirt Blocks will randomly be replaced with Dirtiest Blocks

Tips

 * Dirt Blocks are without doubt the most accessible block type to new players, making them useful for building a rudimentary shelter to take refuge during the first night.
 * Replacing dirt with non-corruptible blocks during Pre-Hardmode near your base will help a lot to prevent any possible contamination from The Crimson or The Corruption.

Trivia

 * In some of his quotes, the Merchant claims to sell dirt, despite not actually selling it.