Strikers only have punch and kicks attacks, no grapples (Bullies are an exception).
Melees are like the strikers, but they can grapple. However they have generic grapple attacks (g_melee is an exception).
Grapplers are like melees but they will try to grapple the opponent more often instead of punch or kick. They have special grapple attacks (P_grapple is an exception as they have generic grapples). Also their block frequency is higher than any other fighter class.
Im not sure about Ranged since there is only 1 ranged fighting style (N_ranged). Well there is also the unused fighting style called G_ranged but I don't see anything similar.
I think generic is only for the player.