Magic is like an extra pair of hands, for me, at least. It's useful, of course, but people can coil it into a fist and knob another bloke in the nose with it, too. Of course, dark magic's known to influence people as well, at least, according to Sirius and some of the other adults.
But power levels aren't something I'd bring into Harry Potter. I'd like to think that magic's more about how you use it, and your experience with it, the myriad of ways that it can be used to perform tasks. Like, say, if we compare in real-life, computers. Voldemort knows programming language, manual virus engineering and the like, while, say, Harry in the fifth year just became aware of C++. Magical cores (Within Wizards/Witches, not objects) were brought up by Madam Rowling, but I don't think we were given an elaborate dissertation of how these apply to the skill levels of wizards.