What does it mean to jailbreak an iPhone? - 2

Someone told me its where you unlock all the apps. Also will jailbreaking ruin your sim because I have an AT&T prepaid sim in an iPhone 3GS

Someone told you wrong. Jailbreaking allows you to download unauthorized software.

By jailbreaking your iPhone you're able to gain greater control over your iPhone and bypass apple restrictions such as you can download themes, lockscreens, even change the booting logo of iPhone. You can use any sim card on carrier locked devices and much more. No it doesn't effect your sim card. It voids your warranty but you don't have to worry about that.

http://en.wikipedia.org/...ilbreaking

Jailbreaking won't break your sim. It can be done by only changing the software on the phone (the operating system will be changed) and will allow you to run 3rd party software on it (pretty much apps that apple doesn't allow you to run on it). Hardware problems like dropping your phone and breaking the screen for example will still be insured if you have insurance on it. If it however stops working because of the new software (not booting up, freezing etc) it's all on you. The chances of jailbreaking actually breaking your phone are slim to none and jailbreaking is completely legal.

Jailbreaking is a modification made to an iPhone that allows you to do things that Apple doesn't want you to do, such as installing apps obtained from sources other than the App Store.

Jailbreaking is for geeks. It reduces the reliability and security of the iPhone, and makes it more vulnerable to hacking and viruses. It can interfere with the operation of many apps and can cause the phone to behave strangely or get stuck in certain situations. If you are an ordinary iPhone user, and not a geek, you're far better off without jailbreaking. And if you had to ask what jailbreaking is, it's a pretty good bet that you're not a geek.