The mage is ranged magic casting damage dealer. Mages deal massive amounts of damage by casting balls of fire, ice, and arcane. As far as casters go, mages can deal the highest DPS. Mages also carry a big punch with area of effect spells. They come with a few abilities that assist the rest of the party such as the ability to create food and water and make portals that will teleport wow gold players to cities for quicker travel.
Mages are great at crowd control because of their polymorph and frost nova spells. They also have slowing effects through all 3 areas of magic- fire, ice and arcane. The heavy damage that mages are able to produce can only be done if they have mana. Without mana, mages become useless. Because of this they are sometimes not as good during long battles like you will find during end game instances. Still, they can provide the raid with huge burst damage and definitely help to get the job done.
Mages are decent levelers, especially when you get past the first 20 levels. They are highly dependent on mana which means there's a lot of downtime between fights but they also kill extremely fast so it evens itself out. Some people use the Area of Effect technique to speed up leveling. This is done by agroing large groups of mobs and rounding them up into a pile, then spamming Area of Effect spells like Arcane Explosion. The mage will typically run out of mana and lose much of their health so they will need to eat and drink cheap wow gold afterwards. There have been videos on the internet showing mages gaining levels very quickly in the level 50 ranges by using AoE techniques.
During end game instances they become useful for multiple reasons. On my server, they are known as waterboys because they give out the precious water to the rest of the casters. Mages also get the spells Detect Magic, Counterspell, and Remove Lesser Curse which make many end game battles much easier. They can also crowd control with sheeps.