One easy way to visualize the differences is to note the number of .357Mag shells that can fit in each:
J = 5 shot (also set up as 6 rounds of .22LR, .22Mag or .32Mag - started life as five rounds of .38Spl)
K = 6 shot (oldest current frame size, started as six .38Spls, also available in .22).
L = 7 shot (started life as a beefier 6-gun than the K for longer life with hot .357s, is also found as a five-shot .44Spl)
N = 8 shot (actually started as a six-gun big-bore in .44Spl/.45ACP (think "Indiana Jones"), was then used for 6 very beefy rounds of .38 and then .357, became the .44Mag sixgun (Dirty Harry's gun was N-Frame) and is now set up for 8rds of .357, 8rds of .38Super or six rounds of 10mm/.45/.44.)