Laws differ from theories because laws do not provide an explanation for how things work or could possibly work. A law describes what happens or needs to happen under certain conditions. A law can predict what will happen as long as those conditions are met. For the purposes of this discussion, a "law" is a rule that has been formalised by repeated testing. It is also a generalisation. A theory, on the other hand, is an explanation for an observation that is supported by a large body of evidence.