What are Laws of Nature?

Scientists, and philosophers, regularly speak of the laws of nature: Newton's laws of motion or Avogadro's law. But what is a law of nature? Is it just a generalization that allows for exceptions? Is it just a summary statement of a pattern in events we have observed so far? Is talk of laws an indirect way of talking about the powers that objects and properties have? Or are laws somehow separate entities that make objects behave the way they do? Do they show us how things have to be, not just how they happen to be? Given what laws are supposed to be, are there really any laws of nature? Prerequisites: PHIL 80, PHIL 150 (or equivalent, and PHIL 180 (or equivalent).