Redsn0w is a program that is used to jailbreak an iPhone or iPod touch. The jailbreaking process is what allows you to run custom application on your iPhone or iPod.
[Updated for iOS 5.1.1 & iOS 6 - Redsn0w 0.9.15b3 ]
UPDATED with Redsn0w 0.9.6.rc16
Redsn0w 0.9.6rc15 can jailbreak untethered iPhone 4, 3GS 4.3.3 firmware. You can jailbreak iPhone 4, and iPhone 3GS 4.3.3 using RedSn0w. No iPad 2 support yet.
Redsn0w 0.9.6rc14 can jailbreak iPhone 4, 3GS 4.3.3 firmware. You can jailbreak iPhone 4, and iPhone 3GS 4.3.3 using RedSn0w. However, for now, RedSn0w provides a tethered 4.3.3 jailbreak solution. No iPad 2 support.
The iPhone Dev-Team has released Redsn0w 0.9.6rc14 to include support (except for iPad 2) for untethered jailbreak on iOS 4.3.2 using the same i0n1c’s exploit which was used to previously untether 4.3.1.