Magic is power, it is the life blood of the world, and while it is sometime viewed with suspicion like among humans, it is everywhere. Magic is pulled straight from the plane known as Arcanum. Everyone and everything possesses an attachment to Arcanum. Through this attachment, whether it is via a natural connection, like sorcerers, or through intense training, like wizards, one can use this to perform great feats. Magic feels a lot like warm water if you ask any magic user. It starts from the center of your chest, where you heart is, where most assume your soul is, and from there a soothing warm sensation floods your body, eventually bursting forth from your hands or where ever it comes out to cast the spell. Now, this causes some issues when casting more powerful spells. As you may have guessed, the more powerful the spell, the more intense this warm feeling becomes. As a spellcaster becomes more powerful, the more resistant your body gets to this warmth. But if you cast spells excessively, this can put a great strain on your body, and if you try and cast out of your league, assuming you are even able to attempt it, you can perminantely harm yourself. The longer you train, and the higher you climb up the latter of expertise in wizardry and socerery, the more "heat" you are able to handle from the casting of spells.
Additionally, since it is not impossible, only potentially harmful, many wizards in dire situations will cast out of their league, unleashing spells of far more power than they can handle. But unfortunately this results in the death of the spellcaster, or in the very least the permanent damaging of their body. When you cast a spell out of your league, and it causes damage to your physical body, this is referred to by magic-users as back-lash, or magical recoil by some. Though this isn't always fatal, it does have the potential to be, and the more powerful the spell you attempt, the more likely it will have a terrible effect.
Over all magic is great gift that mortals have learned to use. It is dangerous, and some view it with distrust and even disdain. But for the most part, it is a useful tool used by almost every mortal race on our planet. Tred carefully though, the life of a spellcaster is one of dedicated practice and endless improvement, but the rewards are just as vast and unending.