I recently started using an Eragon-based magic system. I think the movie sucks, but they have some good ideas. Magic is the manipulation of energy, trough means of some almost forgotten language. You need to state what you want in this language, eg. you can say 'burn that door' in the ancient language and your magic will burn the door. However, stronger magicians require no sentences, but can use words, eg. 'burn door' or 'fire door' or even 'flames portal', as long as it is clear to the magician himself what he means with it. A very powerful magician might be able to, for instance, say 'pig tail sock' and burn a door, as long as the link between the sock and burning the door is clear to him.
Magic is also possible without saying a word, however this is extremely dangerous, as it is guided only by your thoughts. Eg, if you think 'burn the door' and at the same time think of your wife, you may come home to find her charred remains (Or her body with a door trough it. Or a door with her stuck in it. Or a burning door, whatever). Only trained magicians attempt this, and only if necessary.
As said before, magic was the manipulation of energy. Life energy, to be exact. This means, if you want to lift a stone with magic, you would lose as much energy as you would by lifting it by hand. This amount of energy even increases when the stone is further away. Some spells have proven to be so demanding in terms of energy, nobody has ever succeeded, eg. resurrecting the dead or seeing the future. If you cast a spell that requires more life energy than you have, you die, unless you have a way of interrupting the spell.
You can also take life energy from living things (plants, animals, sentient) around you, but this fact, and especially the method to achieve this, is a well-kept secret. So far, no magic-user has ever found a way to use non-living energy (solar, movement, ...).