Get a book
Small Arms of the World (Smith&Smith) covers alot of the operation of common military firearms. It is a great reference to have.
If you really want an indepth reference, The Machine Gun (Chinn) is a multi volume study done for the Navy Dept back in the late 50s (or so) and covers in detail virtually every automatic weapon ever made. This massive work used to cost quite a bit, but I found a copy on CD ROM at a gunshow for $10.
In a nutshell, the AK 47 was designed as a selective fire weapon, and had the internal parts slightly redesigned for sale as a semi-auto. They did the same thing with the M16. Early AR-15 had only a couple of differences inside, but later ones had the internals redesigned to prevent easy conversion.
I don't know enough about the internals of the AK to be able to answer just which parts were changed, and how, but if you get one a good reference, and compare it to a semi auto Ak, you will be able to see the difference.