Spells are magical syntaxes that, once cast, will consume some mana in order to produce the desired effect.
All spells require a certain amount of mana every time they are used, except for Taunt, and have a minimum magic level requirement. In addition to these restrictions, some spells can only be used by certain vocations. Most of the spells require to learn (buy) them before being able to cast.
You cannot use spells too rapidly, each spell, depending on the type, will exhaust you for 1 or 2 seconds. Note that you can not get muted for quickly casting spells, but you cannot cast spells while muted.
The skills of the magic user have a direct impact upon the effectiveness of most spells and runes. While the magic level has the largest impact upon the effect of spells, it is also effected by the experience level to a lesser degree.
Spells are useful for many different things. With spells, you can do almost anything from summoning a horde of skeletons to fight by your side, to generating magical light to illuminate your path.