Like I said....read about it.
Redsn0w 0.9.10b8b will tether jailbreak iOS 5.1 on iPhone 4 or lower.
A tethered jailbreak is where you have to use redsn0w to reboot. If you reboot away from your computer, you can still use the phone for calls txt and web, but Cydia, anything from Cydia, and Safari will not work, until you boot tethered with redsn0w again.
And untethered jailbreak is where you can reboot and keep the jailbreak fully functional.