A rundown I wrote a couple of years ago in response to a question on The Firing Line...
Here's the rundown on S&W letter frame sizes:
I frame - The original hand ejector frame, introduced around 1896. These were chambered in .22, .32 Long (a VERY few were chambered in .32 S&W, and bring SERIOUS collector prices), and .38 S&W.
An improved I-frame introduced S&W's first use of coil springs only a few years before the introduction of the J-frame.
J-frame - After WW II, the company wanted to chamber the .38 Spl. cartridge in a small gun suitable for police undercover use. The I-frame was too small to take this chambering, so the J-frame was introduced in the 1950s.
The J-frame has been chambered in .22, .32 Long, .32 H&R Mag., .38 Spl., and in a slightly enlarged version in the past few years, .357 Mag.
K-frame - The "classic" S&W .38 revolver, introduced around 1899, and over the years chambered in .22, .32 Long & Mag., .32-20, .38, .357, and a very few test guns for the Army in .30 Carbine.
L-frame - As police began to transition to the .357 Mag. for both training and carry (as opposed to training with .38 Spl. and carrying the .357 Mag.) the drawbacks of the K-frame became apparent in that they would, given a stead diet of Mag. ammo, begin to shoot loose.
The L-frame was introduced in the early 1980s to counteract that by beefing up the frame size and weight of the gun, but retaining the K-frames grip dimensions. Grips are interchangable between the K and L frames. The L-frame is almost exclusively .357 Mag. territory, although they have been produced in .38 Spl. on contract (mainly for South American police forces). The 696 in .44 Spl. is also an L-frame.
M-frame - The smallest Hand Ejector ever built, and in production from 1902 to about 1922. These were the original "LadySmith" branded guns. They were 7-shot .22s, and came in three separate models. All three are HIGHLY collectible, and bring serious prices.
N-frame - The largest of S&W's revolvers. These have been chambered in .22 Jet/Long Rifle, .38 Spl., .38-44 Heavy Duty (the original +P .38 Spl. round), .357 Mag., .41 Mag., .44 Mag., .45 ACP, .45 Long Colt, .45 S&W, .44 Russian, .44 Special, several other calibers mainly for Canadian and European consumption, and a few dozen test guns in .30 Carbine for the Army.