it will install this app called Cydia which you'll get lots of cool stuff from but some of the things you might install from Cydia could effect your idevice (your iPhone) it could make it better or worse but there is mainly no problems, but if you want to remove it ; the best thing to do is a restore using iTunes
By jailbreaking it.
Jailbreaking will void your iPhone's warranty.
You cant without jailbreaking it.
Jailbreaking is free.
Jailbreaking a phone is partially legal and partially illegal. I have a post regarding this http://www.techlucid.com/2013/11/is-jailbreaking-illegal-or-not/
Yes and it works pretty well for me
yes it is absolutely legal
slows down ipod/iphone/ipadpossibility of bricking it (rendering it unuseable)its illegal to use pirated contentDefiantly voids warranty
It is okay to jailbreak your iPhone 3g. The only downside to jailbreaking your iPhone is that you will loose your Apple warranty.
Yes by jailbreaking your apple device
No. Sorry.
It is not illegal but will invalidate any warranty.