These are the Smith&Wesson frame sizes - J-frame is their small .38 Special snubby, K-frame is the .38 Special generally but also some .22LR revolvers, L-frame is larger for the .357 Magnum and the N-frame is the largest frame for the bigger .357 Magnum (Model 27/627) and .44 Magnum (Model 29/629).
The other companies, Colt, Ruger, etc - also have their names or designations for frame sizes but arne't as well known as the S&W designations.
