When we talk about the frame of a revolver, we mean the part that goes around the cylinder. It contains the cylinder and the trigger mechanism. The grip frame is generally a part of it too. The barrel attaches to it. It is the heart of the gun, and defines its size. Generally speaking, there are three main frame sizes: small, medium, and large.
The Smith and Wesson factory refers to the frames it makes by letters: Their small frame is called the J frame. They have two slightly different medium frames, the K, which is the smaller of the two, and the L, which is a bit larger. And finally, the large frame is the N frame.
S&W also has a super large frame that I believe is called the X frame. It is made in much smaller numbers than the others because it is made for very powerful cartridges that few people use. Those people are willing to pay a high price for these guns, though.
It used to be common to call these small, medium and large frames the 32 frame, the 38 frame, and the 44 frame, because they were all introduced as 6 shot guns in those calibers. But they have all been made in a vast array of different calibers over the years. Now it is more common to think of the small frames as 5-shot 38 Special or 357 Magnum guns, the mediums as 6-shot 38 Special or 357 Magnums, and the large frames as 6-shot 44 Magnums. However, they are all available in other many calibers, which sometimes means they hold a different number of shots. The super large frames are for cartridges like 480 Ruger or 500 S&W.
Furthermore, all these different frames can all have different lengths and thickness of barrels, and different sights, and different grip frames with different types of grip plates. The small ones can have hammers concealed within a hump on the frame, as well as the normal exposed style. They can be made out of ordinary steel, or stainless steel, and given a wide array of finishes. All this means that two revolvers with the same frame can look VERY different from one another.
BTW, the reason S&W has two different medium frames is that the older one, the K frame, dates back before the 1930's, which is when the 357 Magnum cartridge was developed. For decades, you could only get 357 Magnums from S&W in the large N-frame size, But demand grew for medium sized 357 Magnum revolvers. This was orignally filled by Colt, whose medium frame revolver was a bit larger than S&W's K frame. S&W started making 357 Magnums on the K frame to compete (they still do), but eventually they decided the K frame was not entirely adequate for that ammunition, so they introduced the L frame to handle it in the 1980's. The L frame is pretty much the same size Colt's medium frame was. Somewhat surprisingly, S&W has continued to make both sizes ever since.
Nowadays, even small frame revolvers the the S&W J and the Ruger SP-101 can handle 357 Magnum, so things have really changed.
I hope this helps. Describing the Ruger frame series, the older and newer Colt and S&W frames (for example the S&W J frame replaced the old "I" frame) and how single action revolvers (think cowboys) fit into this would make this post very long and tedious.
Also, it is late, and I may have already made a hash of this. Corrections would be very welcome.