AK-type rifles have two main types of receivers: milled and stamped. These days, the country of origin is usually the determining factor in which type of receiver you have. Distinguishing between the two can be quite a task for someone that does not know what the specific differences are. So, if you'd like to be informed, keep reading.
The first AK-47s that were built had milled receivers. They were machined from a solid block of steel which made them very sturdy, but also very heavy. One of the easiest ways to visually differentiate the milled receiver is the large, rectangular-shaped cut-out above and in front of the magazine well that is nearly 5" in length. This was designed to reduce the overall weight of the rifle.
The trigger guard is stamped and is riveted onto the bottom of the receiver in a very similar way as on a stamped receiver. The pistol grip nut is riveted in place onto the receiver just behind the trigger guard. The magazine well and magazine front latch slot is machined into the receiver. You should also notice that some of the sharp edges above the trigger and magazine have been milled off (chamfered).
Stamped receivers are much more common, easier to build and make yourself, and can be very strong. While milled receivers are machined out of a heavy, solid block of steel, stamped receivers are stamped from a flat piece of thin metal, bent into its box shape, and riveted together with a center spacer, and both a front and rear "trunnion." Stamped receivers can easily be visually differentiated from milled receivers by noticing the small, shallow indentation approximately 1" long on each side of the receiver, directly above the magazine well. This indentation is called the "magazine guide." This is not to be confused with the large, weight-reducing cut-out you find on milled receivers. You can also differentiate the two because of the many external rivet heads on the sides of the stamped receiver.
The trigger guard is stamped, just like milled receivers, and is riveted onto the bottom of the receiver. The grip screw nut, however, is a separate piece that fits down from inside the receiver through the square hole behind the trigger guard. Pistol grips are usually interchangeable between receiver types.
Milled receivers are commonly thought to be stronger than their stamped counterparts. Some authorities also believe that they're more inherently accurate, but there are some who dismiss this difference as negligible or deny it altogether. Despite all of the differing opinions on which receiver type is better, according to Soviet manuals, stamped receivers generally have a longer life expectancy in terms of rounds fired.
There are many different countries that make both types of receivers, so please do not assume that your receiver is milled or stamped based on it's country of origin.