Wizards possess huge damage capability but have very little defense and endurance. They function best as a DPS, dealing massive damage with a variety of spells as well as being able to effectively maintain stun on a single enemy.
Unique Abilities: Edit
In pvp they lose to almost every class in game due the lack of defense, hunters/shades are likely to one shot kill them after you hit level 20 in pvp areas, they will probably not solo bosses and enemies might kill them easily.
Wizards can be more useful in parties as a dps and they can often can one shot kill enemies and players(Well shades will likely stay invisible and will probably take the first shot killing the Wizard.)