http://en.wikipedia.org/wiki/Jailbreak_(iPhone)
So you can change themes ie icons,backgrounds. Change fonts , add different sounds for txt/email alerts etc.
Can also download little apps that add functionality such as video recording, being able to close all your browser tabs with one button press rather than having to click them all, you can download cracked apps(which you mustnt do because its naughty).TBH, a lot of the stuff has been added to the new version of the phone (which cannot be jailbroken at this time)
Its simple enough to do. If you buy one, either google it or PM me and Ill sort out some links for you
