"The biggest difference between a law and a theory is that a theory is much more complex and dynamic. A law describes a single action, whereas a theory explains an entire group of related phenomena. And, whereas a law is a postulate that forms the foundation of the scientific method, a theory is the end result of that same process."
Here's a simple, easy to understand explanation of the difference between a law and a theory in science.
And here is an explanation of the difference between a theory in common use and in scientific use.
I hope these help.