Mages have their own special abilities that they bring to a group. For example, a Conjurer has a pet that does his damage for him, and uses DoT's (Damage Over Time) spells against the mob.
There are subtle differences between the classes. Necro's and Conureres are similiar, but Conjurers spells are mainly fire, ice, and earth, while a Necro's spell line is diesease and poison based.
There several different versions of casters, and your abililties change depending on which subclass you pick when you gain certain levels.
In group dynamics, you will always need a tank, a healer, and a bunch of DPS's (Damage Per Second). There is no one "best class", it all depends on what you like to play and if you are having fun with it.
I enjoy playing my Conjurer, but I have alts that I also enjoy.
Check out http://everquest2.station.sony.com/en/#home for a basic review of the classes.