The most famous AK is of course the AK 47, which fires a 7.62x39 round.
There's also the AK74 which fires a 5.45x39.
Here's the two next to each other. Due largely to call of duty people often assume that the 74 is physically significantly smaller than the 47. This is of course a misconception, both guns come in a variety of sizes. This is the AK74su from CoD:
The 74 was the Soviet response to the nato switch from 7.62x51 to 5.56x45. The idea behind this was that the side that fires the most bullets usually wins. If bullets are smaller, a soldier can carry much more of them. A soldier with an AK47 carries 90 to 120 rounds. One with a AK 74 could carry 150 or more. Even though each round packs less of a punch, the sheer volume of them makes up for it. Also a new thought is that smaller faster bullets are better at armor piercing. Despite the existence of the 74, the 47 of course remains popular across the world.
Enjoy, more next time