A "law" is a readily observable fact about something. It is something that is obvious and undeniable. Allow me to clear up a common misconception right now, laws are not a "higher" stage than theory, and no theory ever becomes a law. Laws are simple and obvious statements about a phenomenon that never require a second guess, or an experiment, to verify them (for example, there is a law that states that there exists an apparent attraction between all objects having positive mass...it's called the law of Gravity, and it's not just undeniable, but it's readily observable and demonstrable (by virtue of the simple fact that you are not floating about, but are anchored to the Earth)).

Now, a "theory" is an advanced hypothesis. An hypothesis is a plausible, testable explanation of how a phenomenon works and/or why it works that way. Once an hypothesis has been tested repeatedly, under a variety of conditions, such that it is sufficient to convince a majority that the hypothesis is probably right ("right", in this context, means that it can be used successfully to make predictions as to how the phenomenon will behave if one conducts the same experiment(s) again), it can graduate to "theory", but it is still tested just as vigorously.

A theory can be "strong" or "weak", depending on the amount of evidence there is that agrees with it, the amount of accurate predictions it's made, and the amount of experiments that have been conducted and have concluded in its favor. However, it doesn't matter how strong a theory gets (you might think of such as examples as the theory of Evolution, Relativity, Quantum Mechanics, &c), it never becomes a law. That would run contrary to the definition of "law" as readily observable and nor requiring experimenation for verification. Also, a theory may always be disproven, but it must then be replaced with a better theory.