Guide:Maintaining world purity

Prior to the activation of Hardmode, which is activated by defeating the Wall of Flesh, there lies only a few, relatively small areas of Corruption (or Crimson) on a world. However, once Hardmode is activated, two large strips of both Hallow and Corruption/Crimson appear cutting through the entire world diagonally, reaching from the Underworld to even Sky Islands in its path. Not only that, but the Hallow and Corruption/Crimson generated (as well as existing areas) will spread rapidly through any grass, sand, and stone nearby, and given enough time, both can consume the whole map.

Dealing with the strips that generate upon defeating the Wall of Flesh would be difficult and time consuming on its own, but to get the hardmode ores you would have to indirectly help the spread. Upon smashing a Demon/Crimson altar with the Pwnhammer (Or more powerful tools later on in the game), there is a chance of a single block of your world turning Hallowed or Corrupt/Crimson in a random place, which given enough time, will spread to form new, large areas to deal with.

Corruption/Crimson
In Hardmode the Corruption (or Crimson) spreads far more rapidly and is no longer stopped by mere sunflowers. It will corrupt everything in its path, besides the Hallow and non-corruptible blocks such as bricks and wood. It also has the potential to utterly destroy the Jungle and Underground Jungle because the "evil grass" will turn Mud into Dirt on which Jungle cannot be regrown.

The Corruption and Crimson are fairly similar and work in the same manner.

The Hallow
Despite its friendly looks, the Hallow is only slightly less dangerous than the Corruption or Crimson. It spreads almost completely in the same way as the Corruption except that it cannot spread through the Jungle. Because of this, many players consider it the "light" Corruption. However, if you do not want to take action for keeping your base pure, Hallow is a better choice than Corruption/Crimson, so you may want to plant some Hallowed Seeds. The Hallow does not uproot or prevent native forest plants from growing, nor does it prevent NPCs from moving in as opposed by the Corruption/Crimson.

Details

 * Both the Hallow and Corruption/Crimson cannot convert each other. So corruption surrounded by hallow will not spread, and likewise the other way around. (Tested in 1.2.4.1, Crimson is converted by Hallow; not sure if it is a bug or not.)
 * Neither biome can convert or go through any block except grass, stone, sand, and ice. So digging up a strip of dirt and replacing it with, say, wood would stop its spreading. The grass can spread up to normal grass up to 5 blocks away and the stone, sand, and ice variants can only convert up to three blocks away.
 * If you wish to save your Jungle, spreading Hallow around it as a barrier will prevent it from being corrupted.
 * The Dungeon can completely prevent the spread of Hallow and Corruption/Crimson if it reaches down to the Underworld. If it doesn't reach that far, it will at least slow it down.
 * Floating Islands CAN be turned Hallowed or Corrupted during the initial generation from Hardmode, or if converted blocks are placed near them.
 * The Hallow can convert straight to Corruption by the Corruptor's Vile Spit projectile.
 * When Plantera is defeated, the Corruption/Crimson and Hallow spread will be slowed down to 33% of their speed.

Containment
If you do not wish to have your world turned into just Hallow and Corruption/Crimson or even if you want to completely purify your world, containment is key. A simple and also the best way of containment is making straight tunnels from the surface down to the Underworld (commonly called Hellevators) that are three block wide. An easier way would be to place block from the opposite biome of what your are containing around it(Example: Surrounding the Corruption with Hallowed) to stop the spread. The obvious drawback to this plan is that you will have to deal with the biome you're using to contain the other biome with.

Protecting the Jungle
Since the Corruption/Crimson cannot spread past Hallow, and the Hallow cannot spread past the Jungle, you can line the edges of your Jungle with Hallowed blocks to prevent damage to it. This will however only prevent surface spreading.

Pre-Hardmode
Since the Corruption/Crimson spreads like crazy after Hardmode activates, you may wish to thin out some of it to limit the spread. Using some Purification Powder (cheap) is extremely useful and can clean up to 48 blocks when thrown. Start in the Corruption chasms, throw the powder at the ebon/crimstone and mine the normal stone. Then, you can use the stone to make statues (armor or text, however armor is usually easier to get) which you can sell for money, which in turn you can use to buy more powder. You lose a slight amount of money doing this, but the monsters you encounter drop tons more which you can use also. Cleaning away evil and making a small bit of money! Surely that can't be bad, eh?

Complete Conversion
To completely purify, cleanse, or corrupt your entire world you will need the Clentaminator. If you wish to spread Hallow to all of the world and stomp out the Corruption (and same other way around) you can buy the Blue Solution(For spreading Hallow), Purple Solution (For spreading Corruption), or Red Solution (For spreading Crimson). If you want to purify your world (Remove all Hallow AND Corruption/Crimson) you would need Green Solution All you have to do it spray the device over all unwanted blocks until the biome you do not want is completely gone. Much easier said than done.

The best way to completely convert or purify your world is to dig Hellevators every 120 blocks away, glide down each one using wings or slowfall, and spray both sides. A drawback to using the Green Solution is that all the glowing mushroom biomes will be converted into Jungles. An easy way to fix this is to go back to each pocket of Jungle that was glowing mushrooms and spray all around it Dark Blue Solution from the Truffle. Because the Clentaminator also produce a decent amount of light that can completely go through blocks, you may want to make Hellevators every 80 blocks instead to completely map out your world, however with slow light.