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 ]
iPhone Dev Team has released Redsn0w 0.9.15b3 for jailbreak iOS 6 Tethered with having Cydia and ability to restore to older firmware not being signed by Apple for all A4 and A5 device like iPhone 4s , iPad 3 if you don’t upgrade to iOS 6 .
Redsn0w update to 0.9.10b5c Version b5b fixes an issue with using custom ramdisks on iPhone3G and iPod2G, and version b5c prevents redsn0w from crashing due to the ever-growing ramdisk size.
Corona update to v1.0-6
It has been nearly eight weeks since the first declaration by pod2g that he had found a bug that he believed could provide an untethered jailbreak for iOS 5. We find ourselves exactly fifty days down the line, and what I imagine has been a lot of sleepless nights for the developers involved, but we now officially have the release of an untethered jailbreak for all A4 devices running iOS 5.0.1.