Rooting is just giving extra permissions to the phone that your wireless company doesnt want you to have. Android by itself is open source, meaning tbey want people to play with things like custom roms and porting. Rooting does void your warranty no matter what anyone tells you. If you look at your wireless contract it says your not allowed to mess with system files.
You can unroot by reupdating your phone or there are some unroot tools out there like super one click but you have to make sure its compatible with your phone.
Rooting lets you do stuff like overclock/underclock(processing speed) custom roms( gives you newer android versions that you typically wouldnt get, and makez phone work better) and you can do cool stuff like game emulators which let you play old nes,n64, sega games on your phone.
Just head to your phones forum and theyll have a section explaining root and all its wonders
Turn that frown into a murderous grin