Hypothesis - an educated guess that may or may not be backed up by facts. When most people say "in theory" they really mean "I hypothesize that...".
Theory - A way of explaining why something works the way it does, backed up observational/experimental results. The Chilton's guide for my car is the theory manual behind the operation of my car. The theory of gravity attempts to explain why gravity works the way it does. A book on computer science is the theory guide explaining how your computer works. And so on...
Law - A description of a phenomenon, but does not explain the "why". A train schedule is an example of a "law". It explains when the train will arrive but has no power to explain why the train got there on time. The "law" of gravity explains why your head hurts when someone drops a brick on it, but only the "theory" of gravity explains why the brick dropped downwards, instead of just floating there in space. Last but not least, in case it is not obvious, theories are never promoted to "law" status.
Hypothesis are educated guesses. Theories explain "why". Laws explain "what".
