This guide is designed for beginner players, who do not understand much of the game and want to learn more. Besides, there are hardly any well-done guides, which encourages me to make one myself.
In Terraria, there are 2 vile biomes, corruption and crimson. There can only be one in your world, and they are called this way since these biomes expand slowly throughout your playthrough, and will not stop unless you try. Some ways to stop its expansion is: The Druid´s purifier dust (Npc that moves into your world when you have defeated a boss), or the * Clentaminator * (Weapon that sells the Steampunker (Npc that moves in when you have killed a mechanical boss) for two platinum (200 gold).
The corruption possesses weak enemies compared to those of crimson. In the corruption you will find flying monsters called "Eaters of Souls", and a stronger and faster normal worm alternative. The crimson, on the other hand, has an alternative of the soul-eaters, called "Crimeras". There are also face-monsters that are the same as zombies but stronger and tougher. Also, inside the caves you can find spiders called "Blood-Creapers", which are extremely dangerous for new players.
The terrain that presents the corruption is flat with very dangerous vertical caves that will kill you if you are not prepared. if you go down enough, you will find a horizontal gallery that expands throughout the entire biome. The crimson, on the other hand, has a rather irregular terrain, with caves with safe entrance that will lower you (diagonally) more and more until you reach a huge sphere that also has other minuscule caves that expand along the biome .
Orbs and Hearts
Within both caves that possess these biomes, the orbs of corruption, and the hearts of crimson are very common. These can be destroyed by bombs or by breaking them with a hammer, but they will be surrounded by the stone of the biome, which cannot be mined with the pickaxe you start with, or with any made with minerals. You must break them either with bombs, or by dusting them from the previously mentioned Druid´s dust that will turn the corrupt stone into normal stone, which obviously can be mined perfectly.
Breaking these orbs or hearts will drop weapons, accessories and pets that will become of great use to kill some bosses. I must mention that breaking 3 of these orbs or hearts will spawn the corresponding boss of each biome.
Both biomes have bosses, in the case of corruption is the Eater of Worlds, and in the case of crimson is the Brain of Cthulhu. The EoW behaves like an extremely strong worm, which has a head, 48 parts of the torso, and a tail. If a part of the torso is destroyed, the EoW will be divided into two, three, and so on until he is defeated. This boss will drop ore from corruption and scales, which allow you to create the pickaxe that allows you to mine Hellstone and the shadowy armor.
The brain, on the other hand, cannot be attacked before you kill its minions, who teleport and follow the brain. After you defeat all his minions, you can attack the brain, which I have to say has less life than the Eye of Cthulhu.
The brain will try to attack you by teleporting quickly but predictably. After you defeat him, you will notice that the minions dropped crimson ore and the alternative scales that would allow you to create, the pickaxe, and the corresponding armor.