Crimson



Crimson is a biome, introduced in 1.2. When creating a world, there is a chance this biome will replace the Corruption. It is similar to corruption in the manner it is generated: stone is replaced with Crimstone, as well as sand with Crimsand, and there are deep chasms which contain Demon Hearts that replace the Corruption's Shadow Orbs. Also, in the Crimson, one can find Crimtane Ore (as opposed to Demonite Ore in Corruption) and Crimson Altars which work like Demon Altars.

Spread
It seems that Crimson spreads exactly like a corruption. Additional test required.

Crimson desert
Crimson can overlap a desert when generated. The desert then becomes Crimson desert. The enemies here are same as in the corruption. (untested on hardmode) Crimson can also overlap with snow biome, creating red snow and red ice.