ak-47 is basically what we refer to as the "type III milled receiver" variety (although there were two previous models of ak-47's, these are so rare in the united states as to ignore them). these are chambered in 7.62x39, the soviet m43 round.
in 1959, the soviets adopted a stamped receiver version of the kalashnikov. although folks often refer to it as an "ak-47", thet are more accurately refered to as an "akm" model. these are also chambered in 7.62x39 and featured many updates and revised parts to improve and lighten the old ak-47.
in 1974, the soviets rechambered the akm to 5.45x39.5, the modern m74 round. again, the update involved taking the akm and updating it to improve performance and effectiveness of the rifle.
the sar-2 is a romanian version of an early, often calld "transitional" russian ak-74. it lacks some of the russian features and is perhaps more closely described as a rechambered akm.
currently, it is the closest production ak-74 style rifle to the russian that is available to us without going to a custom build...such as the ones by ohio rapid fire, troy selaars at inrange and others. these 'semi-custom' builds consist of a bulgarian ak-74 parts kit that is assembled on an american (or sometimes an imported) receiver. the tradeoff is that you get authentic bulgarian ak-74 parts which are a close copy of the russian originals on a receiver that is not as authentic as the romanian sar-2 receiver (unless an imported receiver is utilized in the build). sadly, the american receivers are not only lacking in cosmetic detail, they are not as sturdy as the imported ones (although you can send your american receiver out for heat treatment to a third party).
clear as mud?