The Apple iPhone currently has an exclusivity agreement with a sole cellular carrier in the United States that essentially locks the phone at the time of its manufacture for use on that carrier's network. However, the iPhone is perfectly capable of being used on cellular networks with similar infrastructure when it is unlocked. Unlocking your iPhone can be a handy procedure to know if you wish to change cellular carriers, but retain your current phone, or if you are traveling abroad and want to use the SIM card from a cellular carrier that is based in another country.
Use Blackra1n to unlock and jailbreak your iPhone in one fell swoop. Only a few steps are required to use this software utility to unlock your device. Upon starting the Blackra1n desktop program, you simply connect your iPhone to your computer using its USB cable and click the "Make it ra1n" button, which is the only interactive feature of the Blackra1n program. A few seconds later, your iPhone will automatically reboot and it will be unlocked.
The program Ultrasn0w unlocks your iPhone after you install it from the third party downloads and extensions manager named Cydia. After you jailbreak your phone, Cydia is automatically installed. Once you add the repository location for Ultrasn0w, you can download it to your phone as a package. It automatically installs itself after it has finished downloading and then prompts you to restart your phone, after which point you can use a Boost Mobile SIM card.
You can also have your iPhone unlocked by using iTunes, but it requires the participation of your cellular carrier to authorize the unlocking process. Your device's IMEI number, which serves as a unique identifier for your iPhone, is submitted by your carrier to Apple's servers. The next time you connect your iPhone to iTunes, the iTunes program will prompt you to install a new "carrier update," which will unlock your phone when it has finished installing.