Crimson

Crimson is a biome, introduced in 1.2. This biome can randomly replace Corruption in a world when it is created. It is similar to corruption in manner it is generated: stone is replaced with crimson stone, as well as sand with Crimsand, there is crimson grass and there are deep chasms, which contain big pulsing hearts, which replace Shadow Orbs. When smashed by a hammer, they work like Shadow Orbs with the exception that they summon Brain of Cthulhu instead of Eater of Worlds. Also, in the Crimson, one can find Crimtane ore and Crimson Altars which work like Demon Altars

Contents
To be expanded

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)