What is the difference between a AR-15 and a AK-47?

The first two posters did a excellent job answering your questions. I just want to add, the ar is more accurate, but that comes at a cost. The ak is more reliable (less prone to jamming, especially when dirty or neglected). If you want to be able to shoot flies off the target at 100 yards, you need an ar. If you want to be able to dig your gun out of a mud puddle and empty a mag without a jam, you need an ak. That is the biggest difference.