After the release of iOS 5, an untethered jailbreak and unlock for iPhone 4 called FastRa1n 0.5 for iOS 5 has been claimed to be released by FastUnlockiPhone.com.
It would allow users to download thousands of applications and modifications from Cydia and use any carrier worldwide. Cydia is a third-party software application for iOS that enables a user to find and install software packages (including apps, interface customizations and system extensions) on a jailbroken iPhone, iPod Touch or iPad.
The new version of FastRa1n is said to be capable of performing an untethered jailbreak on every Apple device, aside from the iPad 2 and iPhone 4S.
Jailbreaking gives you a way to install software on your Apple device that originates from somewhere outside the company's App Store universe, without Apple's seal of approval. It also enables users to make unauthorized tweaks to the device's mobile operating system. Jailbreaking voids the user's warranty and could cause damage to the user's device if not done properly.
As soon as iOS 5 came out, some jailbreak developers released a tethered jailbreak version for various iPhones, the iPad 1 and several iPods. A tethered jailbreak means that if you install it, it will only stay active until the next time you reboot the device. Therefore, once you reboot the device, you will need to redo the jailbreak.
So jailbreakers have created an untethered jailbreak solution for those devices. As far as features go, untethered jailbreaks are similar to tethered ones. An untethered jailbreak allows the device to be powered without computer assistance.