The word "law" is used a lot in science -- the laws of thermodynamics are an important part of chemistry, for example, just as Newton's Laws are central to basic physics. To scientists, a law is both a different and stronger kind of statement than a theory. A theory may seek to explain why reality works the way it does, while a law describes the way it has always been shown to work.

Laws

A law is a description of an observed phenomenon -- a facet of the reality we see around us. Newton's law of universal gravitation, for example, says that any two bodies with mass attract each other, and it mathematically describes how much pull they exert. It doesn't explain why reality works this way, nor offer any conjecture as to why that might be. It merely observes that this is the way things have always been shown to work.

Criteria

In order for a scientific idea to be a law, it must have always held true on observation. In other words, every time you do an experiment, or make an observation, the law must hold true or it is not a law. Newton's universal law of gravitation, for example, has always held true wherever we've looked in the universe. Bodies with mass always attract each other, and the mathematical equation derived by Newton describes how much pull they exert on each other. Since we've never seen a case where this law did not hold true, we can call it a law.

Conditions

Some laws only hold true under certain conditions specified by the law. The ideal gas law, for example, is a mathematical relationship that applies only to ideal gases -- gases composed of molecules that take up no space and that do not interact with each other. You can mathematically show that the ideal gas law must hold true for an ideal gas. In reality, of course, ideal gases do not exist -- all molecules take up space and interact with each other to some extent, so the ideal gas law is just a very useful approximation for real gases. But it is still called a law because you can mathematically prove it must hold true for the conditions where it's intended to apply.

Theories

Another important word you hear a lot in science is the word theory, which is a unifying explanation of the way reality works that has substantial supporting evidence. A theory has been repeatedly tested by experiment and shown to be consistent with the evidence. Theories do not generally become laws, however, because they are two different kinds of statements. A law tells you how reality works or has been observed to work, while a theory tries to explain why it works that way.