Well, it's mostly about harnessing the underlying energies of the world around us and bending them to new ends... Changing it into matter, or kinetic energy, or bending reality, for example.

But it's also possible to tap into the power of particular gods or spirits or demons from other worlds with invocations or prayers. Obviously, that one sounds like it has more potential for trouble, but you'd be surprised. The power to literally bend the world to your will can be incredibly... intoxicating, if you don't have the proper respect for nature and the balance that supports it.