Every day, more and more users who don't want to be limited to use iPhone carriers (i.e. Verizon and AT&T in the US) unlock their iPhones. After you do an iPhone unlock you are able to use whatever SIM card you like with your iPhone. Unfortunately, you can't unlock iPhone just by entering a secret code, which is a common action to unlock other cell phones. Instead you should use a special tool or software to do the unlocking for you.
Although you could find different kinds of iPhone unlock software (paid or free) on the web and hundreds (if not thousands) of websites and articles dedicated to this subject, but you should be super careful to use them or take action based on their advice, because most of them don't work properly (or at all) and could easily result in your iPhone to be damaged and its warranty to be voided.
After you download the iPhone software for unlocking you could also jailbreak your iPhone, meaning that you could install and run your favorite iPhone games and apps even if they're not listed in the Apple's App Store.
With the help of this tool, unlocking your device is very easy and could be done in a few minutes. It supports iPhone 4, 3GS & 3G, and you could happily use it to unlock and jailbreak more than one device, and with any firmware version.
You could be sure that iPhone jailbreak won't damage your device in any way, and while keeping the original options of your phone working (such as using and downloading from the App Store), it only enhances the usability of your iPhone. Moreover, jailbreaking is totally legal.
But remember that it's always better to backup your iPhone. So in case you change your mind or face problems while doing jailbreaking or unlocking you could always bring back the factory settings of your device by connecting it to your computer and clicking the "restore" option after running iTunes.
