The M16 or current model AR15 or M4 that have the collapsible stock etc.. versus the AK47. The M16 was developed in the 1960s for first use in the Vietnam War by the USA. It is a lightweight Aluminum constructed Rifle with a chrome lined barrel that is more accurate than the AK47 but less reliable when put into combat situations when the gun can get dirty. Because of its design the M16 (AR15, M4). The .556 bullet which is essentially the same as a .223 bullet is smaller than the 7.62×39 round that the AK47 fires but again it is very lethal in that it travels much faster and more straight than its AK47 counterpart. The M16 (AR15, M4) is a more modern design than the AK47 and it looks more modern as well. Modern versions of the M16, the M4 have the capability of holding multiple accessories (Laser Sights, scopes, flashlights) that can be put on this rifle with additional Picatinny rails etc.
Now as far as the AK47. It is by far the most popular rifle in the world. You could know nothing about guns, and you would still have heard about the AK47. It was developed in 1947 by a Russian designer named Mikhail Kalashnikov and is still being used around the world. It gets its name from its designer and the fact that most of them are either automatic or semi-automatic rifles thus Automatic Kalashnikov built in 47 is how the name came about. The AK47 is known for its high reliability and simplicity even in the dirtiest war conditions although it is not nearly as accurate as the M16 and its derivatives. The gun puts out the larger 7.62 x 39 round that is close to a .308 size shell. The AK47 is a gun made out of steel with many having wood stocks and other components for looks. The AK47 is also has a gas piston mechanism which keeps the powder residue from building up on the insides of the chamber and firing pin etc thus keeping it cleaner than the M16. Both of these rifles are together the two most popular military weapons of all time.