With 1 account you can make as many characters you want, on as many servers you want.
You can access your account on any PC, as long as the game is already installed.
every class has it's own specific features. Mage has strong spells that do damage on an area, has a couple strong self-protection spells, he can teleport himself a few feet away, he can make food and water for himself and his party, etc...
all mages have both fire and ice spells, they usually just pick 1 type of magic so they can be more specialised and have access to more powerful spells of that type. You can be a fire/ice mage hybrid, but I doubt you'd be as strong as a fire mage or ice mage.
It doesn't matter much which you pick though: you can always respec (that means you can go to a trainer, and he clears the skill points you spent on the skill trees, so you can use those points again on other skills instead) at any time, which is good if you ever decide you don't like being a fire mage and want to change to ice instead.







