Before Hardmode there isn't really much to do. But you don't really have to worry about the corruption in Pre Hardmode! You're best option is making a Hellavator that is 3 blocks long. A Hellavator is a giant hole that leads all the way to the underworld. Hellavators make it take longer for the corruption to get to the other side. You can also get the dryad by defeating a boss which sells purification powder. It is still not a good option though. Also before you get to hardmode you should make Hellavators around you're pylons and your jungle. You get to Hardmode by defeating the wall of flesh. You summon the wall of flesh by throwing a guide Voo Doo doll into lava in the underworld. You get one by killing a Voo Doo demon which is pretty noticeable.

A Voo Doo Demon

During Hardmode the corruption is actually a threat. But after Hardmode you get amazing tools to deal with the corruption. Also when you kill the wall of flesh and get into Hardmode there is another Biome that spawns called the hallow. If the hallow gets to your Npcs they'll still live in their home but some Npcs don't like the hallow and you'll also need to buy a hallow pylon instead of the old pylon when the hallow spreads to your pylons. The best way to deal with corruption is to use the Cletaminator which the Steam Punker sells for 2 Platinum. To spawn the steam punker you need to kill a mechanical boss called the Destroyer. You spawn it with a mechanical worm which requires

• 6 Rotten Chunks/Vertabra.
  
• 6 Souls Of Night.
  
• 5 Lead/Iron Bars.

The Cletaminator takes solution to work that the Steam Punker sells. If you use the right solution you can also spread the hallow or the corruption!

The Cletaminator

The Destroyer

Plantera

Also there is a boss called Plantera which when defeated will slow down corruption spread by 50%. You can summon plantera In the underground jungle after you have defeated all 3 mechanical bosses. There are three mechanical bosses. The Destroyer, Skeletron Prime and The Twins.
The way you summon it is you find a Plantera bulb in the underground jungle, and break it. I couldn't find any good looking images of the bulb so I can't show it.

I hope you liked my guide! If you have any feedback leave it in the comments!