If you have an Iphone but hate your service then you may want to unlock your phone to work with a different provider. Jailbreaking is the process of unlocking your phone to work with any network such as ATT, Verizon, Sprint and so on. According to the FCC they have made this practice legal and many of the cell phone companies have been fighting it to prevent the practice.
Of course if you do this to your phone you void your warranty so just be aware if you do this.
Anyway don't take my word for it. Click here to read the article on Jailbreaking your cell phone.