The links on the left will lead you to pages that describe class capabilities that you should be aware of as a mage. Knowing durations, mana/rage/energy costs, and cooldowns can let you turn a fight around.
For example, a warrior is sitting in your frost nova with a shield out and 10 rage. Should you frostbolt him or will it get reflected? If you know that Spell Reflect costs 25 rage, it's a safe bet you can frostbolt his face.
As another example, suppose you want to polymorph a shaman. They just put a grounding totem down. If you know shaman, you know you can kill the grounding totem (which has a 15 second cooldown), then start to poly, get Earth Shocked, then poly them again before their 5 second Earth Shock (6 if untalented) cooldown is up. Even better, you can fake a poly, cancel it midcast, get them to earth shock nothing, and poly them again, but I digress.
If you're having trouble with a certain class, study it. Find its weakness and exploit it.