Difference between Redsn0w and Ultrasn0w to Jailbreak iPhone 4S

If you are new users of the iPhone, you might want to jailbreak your iPhone. You must select a number of suggestions from friends and colleagues. You have to come up with a name like redsn0w and UltraSn0w. So what is it true? What is the difference between redsn0w and ultrasn0w?
Jailbreak iPhone 4S � Redsn0w
Redsn0w jailbreak software tool launched by iPhone Dev Team jailbreak using the boot ROM