Corruption and crimson only one of these biomes spawns in your game this is where you can make meteors fall if you saw my last blog. They spread like a virus in your world until it is consumed and in hard mode it spreads faster in the corruption there are purple orbs you a break to spawn the boss the eater of worlds, but in the crimson there are hearts if you break them it spawn the brain of cthulu you have to break three to spawn and there are some really good gear like your fist gun that unlock the arms dealer npc. These are very similar there is another biome but its a fairy tail version called the hallow pixies and unicorn attack you it really morbid but back on track these biome are dangerous i wouldn't go in unless you have iron silver or gold armor.