Sorcs have good AEs (or AoE's, area of effect spells) and good DDs for single target take-down.
Summoners have good buffs, some stuns and roots (minor crowd control - the stun has a long recast I think). They have lesser DD's and area damage spells. Pet can tank if you learn its limitations - nuke or do pet-heal too much, and the target attacks you (if soloing).
Enchanter - dps is okay w/ damage spells. They haste meleers and breeze casters (ups power regen, but has a slow recast and doesn't last long). Yes, they stun. And they can mez. Big controversy atm as to whether or not they should mez much. Suprise adds, definitely. Every fight? It depends. I like it when an enchanter is in the group and is allowed to mez a lot. Others hate this. If the group has sufficient AoE power and isn't in a dicey spot - AoEing works. What is an enchanter's place in such a group? Backup, some dps - stunning etc. Are enchanters really needed then? I truly hate to say it, but perhaps not. A lot of players I know dropped their enchanter - couldn't get groups. There are seriously anti-enchanter players out there - if you team w/ old EQ veterans they tend to want an enchanter. FF and other game veterans tend to hate them. Roll an enchanter at some risk of having a hard time finding groups.