I had a few ideas about the different roles (or categories) that magic users could fall into. Personally i came up with 7:
Supporters
Can heal allies, provides stat benefits to said allies and weaken enemy forces. Supporters also can cast protective barriers.
Abilities: Healing Spells, Resurrection Spells, Buffs, Debuffs, Auras and Barriers
Bombardiers
Uses magic to assault either individual targets or groups of enemies from a distance.
Abilities: Pin Point Spells, Area Effect Spells, Spells that attack adjacent targets, and Spells that have damage over time effects
Conjurers
Summons various creatures from wildlife, elementals to demons and skeletons to fight for him/her.
Abilities: Summoning Spells
Diviners
Scouts, in a sense.
Abilities: Farseeing Spells and Detector Spells
Illusionists
Either to project illusions to deceive the enemy or to cast invisibility spells to your numbers.
Abilities: Hallucinates and Cloaking Spells
Mage Killers
The sole purpose of Mage Killers is either to suppress, hinder or eliminate enemy mages.
Abilities: Spell Steal, Spell Break, Mana Burn, Siphon Mana, Silencing Spells
Augmenters
Augmenters use their magic to enhance their fighting abilities or to assist in keep them alive
Abilities: Weapon Magic, Armor Magic, Shield Magic, Body Enhancers
I'll go into more detail in the future.