Corrupted Dirt can be found on the Corruption layer deep underground on every Creativerse-gameworld, shaping the ceiling, the bottom and also walls in between together with Corrupted Stone and strands of Corrupted Wood.
Alternatively you can also "purify" Corrupted Dirt by placing a Healing Beacon close enough or by throwing Purification Bombs and turn Corrupted Dirt into simple plain Dirt blocks by that. These can easily be mined with basic Excavators then.
If blocks of Corrupted Dirt are placed against blocks of Wood (except for Logs), this touch will slowly corrupt Wood to turn into Corrupted Wood. As Corrupted Wood can also corrupt other blocks of Wood touching it, the whole trunk of a tree (including Moss- and Vine-covered blocks of Wood!) can become corrupted after a while, while Leaves, Flowers and Beeswax will stay intact.
Corruption usually "spreads" this way a lot faster if player-characters are close and might come to a stop if player-characters leave the area.
In earlier versions of Creativerse, Corrupted Water and some solid corrupted blocks were able to corrupt simple Dirt to become Corrupted Dirt just as well by touching. This is not longer the case (as of R29 in February 2016).
On blocks of Corrupted Dirt all kinds of (aggressive) corrupted creatures and also occasionally Diamond Treasure Chests can spawn in darkness (where neither sunlight during ingame-days nor artificial player-made lighting hits).
This also holds true for artifical biomes or "arenas" made by players wherever they like, as long as a large enough amount of Corrupted Dirt blocks are put together (usually large platforms of only 1 block "height" will already do fine for the purpose of "spawning" corrupted creatures and treasure chests).