Ascientific is an explanation for how something occurs. it is supported by a great deal of evidence.

law or theory and why?

Law, since "It is supported by a great deal of evidence. A theory is almost like an educated guess but a little more complex. Someone can have a theory and try to predict the outcome, which could be how they want it to go or how they don't want it to go. With a law, the scientist is basically certain of the outcome due to the evidence that was provided when the law was created. 

You can look it up online and some images should pop up