AK-47: 7.62x39mm. The original Russian assault rifle, the cartridge was a compromise, cheap to manufacture, simple to care for and operate, robust, effective. Limited range compared to other calibre‘s. Many built under license from satellite countries in many forms.
AK-74: 5.45x39mm. Replaced the AK-47, followed the theories of a smaller calibre bullet causing more damage to people, etc... More ammuntion for less weight, etc... Robust as the predecessors, many variants, etc...
AK-101: 5.56x45mm NATO. Designed for world‘s weapons markets. Similar in features to AK-74.
AK-103: 7.62x39mm. A return to the original calibre. More powerful than AK-74 with newer technological inclusions.
AK-102,104,105: Compact version for commercial sales outside of Russia. All 3 calibre‘s represented and better characteristics than AKs-74U.
AK-107,108: New version of AK w/ opposing gas rods (reduces full auto recoil) in either 5.45 & 5.56mm.
AN-94: 5.45x39mm. Slated to replace the AK-74 from service, but due to primarily economic factors, has seen limited release internally in Russia (mainly elite units). Considered the "professional‘s" choice. Very accurate w/ 2 rd burst. Not likely to be seen in developing countries.
That‘s the roundup. There are so many variants, it‘s a very popular family of rifles. Due to it‘s low cost and ease of operation. Not my first choice, but then again, as so many US troops are doing, it‘s abundant, so why not. In Iraq, there are reports of US troops using AK variants as ammunition is plentiful and the robustness comes in handy in such a sandy theatre.
Hope you find this useful.