I'm very suprised we haven't posted this video before, but I took a gander through the archives, and while we've touched on the magic of AoE mages before, I don't think I've seen it. If you want to know how to hit up a whole bunch of mobs at a time while playing your mage, this extremely thorough guide will teach you top to bottom.
And for everything else you want to know, you can check out also very in-depth text guide by Bangy posted over on the EU forums, who also references the video above. He covers everything, from spec to process to the best spots to do it at every level. Mage AoE is extremely powerful when set up correctly-- for grinding in the right places, it almost breaks the game. There are a lot of compromises that have to made in terms of how and where you can do it, but in the right place with the right abilities, there's no faster and easier way to crush whole groups of mobs at a time.