AK-series and variants explained by me, to the best of my ability.
The Ak47/Ak74/AKM are different weapons, from different time periods. AK47 was the weapon of choice in the Russian military in the 50's, and the AKM was in the 60's. The AKM is a "upgraded" version of the Ak47, but still a different weapon. Then in the late-70's, they adopted and used the AK74. It fired a different round, the 5.45x39mm which was deemed more proficient at, well doing what guns do, killing. (however, this change was likely made due to the fact that this round is cheaper to make.) At this point I should acknowledge the whole "What does adding a U to it mean? What about an S? And what's an Ak74M?" An Ak47/74 S simply has a folding stock on it. An AK74U (there is no Ak47U) is a shortened carbine version of the Ak74. The AK74M was developed to be a "modernized" version of the AK74. This weapon is likely to be known as a "Black AK" or something of the like. It is what the Russian Military uses today. However, you may also know of the AK100-series which is more confusing. The Ak101-105 still mean "Avtomat Kalashnikova" however, in the 107-108 the "AK" means "Alexandrov/Kalashnikov" representing, Youriy Alexandrov, the man who designed the weapons and Mikhail Kalashnikov whose weapons he took inspiration from. The AK107/108 were designed to be cheaper versions of the AN94, developed by Gennadiy Nikonov, (AN=Avtomat Nikonov) which serves a larger use in the Russian Military than any of the Ak100-series weapons, while still being very limited.
This is pretty much all I know about the AK series.