Magic actually changes the World at a fundamental level. It draws from the very words of creation that were used by the gods so long ago. To combine them and speak them is to create the World anew… or destroy it.
Casters use Words of Power to cast spells. Various combinations allow for various spell effects, ranges, durations, etc.
The Words of Power manipulate Creation as the gods used them aeons ago. To use these words is to recreate and change the world. Magic is as powerful, and dangerous, as you can imagine.
Words that casters know are based on class, school, and language. Not all classes can use all spells, if a caster specializes in a particular school of magic, then they limit what words are available to them as well. But language is also important. For example, an evil spell would have been first spoken by an evil god and that might mean that it is in infernal (for an evil spell of order) or abyssal (for an evil spell of chaos or destruction). This means that some languages and thus words are awfully hard to learn and thus need the investment of resources and time in order to learn them from the most secret of sources, ancient of tomes, or most powerful outsiders.
These spells are then cast by scribing, signing, or speaking the words with the right components with the right timing. A caster might take a manticore quill and carve the words into his own flesh to make his body resistant to poison; eat bat guano in order to lace his voice with the power of fire; or perhaps use mirror shards to slash the words into the air in front of him making an illusory reality.
Want to invest your arm with the potential of the spell and thus cast it more quickly than a full round action? It takes up a higher spell slot to pre-scribe it and tap into it later as a standard action, higher still for a move action, and so on.
But to use such knowledge is dangerous. One wrong pronounciation or flawed gesture could leave one mad, injured, or worse. All casters must make a Spellcraft check against a DC. A natural 20 is possibly a critical success while a natural 1 is possibly a critical mistake.
Critical success means that the spell power is doubled in some way (double damage, double duration, double the area, etc.). But a critical mistake means that the power backfires in some way (damages the caster and/or allies, damages the spellcasting attribute of the caster, etc.) The same effects are possible for either successes or mistakes, only the DM chooses the failures.
Double area of effect
Harder to resist (increase DC by 2)
Double damage (to self or nearby friends)
Half the range (possibly harming friends or innocents)
Wasted spell (lose the word as if cast, but does not work)
Insanity (Int or Wis drain)
Loss of Confidence (Cha drain)