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 ]
Sn0wbreeze also know as PwnageTool for Windows, this tool will untethered jailbreak iOS on iPhone 4G, iPhone 3GS / 3G, iPod touch 4G / 3G and iPad without basebande update , setup custom boot logos, activate your iPhone, set a root partition size, build custom firmware, install SSH, and more.
[Updated for iOS 6.0.1 - 6 - 5.1.1 ]
PwnageTool allows you to restore to a custom IPSW file. For instance, you can restore to a pre-jailbroken firmware while simultaneously maintaining your current baseband (and thus your ultrasn0w carrier unlock). You can also add whatever packages you want in the “Expert” mode of PwnageTool, if you wish to pre-install Cydia packages.
[Updated for iOS 5.1.1]