The difference between "Evolution" and "Theory of Evolution"
Evolution is the observable and active process of change in gene frequency within a population over time.
Theory of evolution is attempting to explain how this works via random mutation, selection, genetic flow and drift.
Think of it as the same thing as gravity and theory of gravity. We observe stuff falling down all the time. It's observable fact. The theory of gravity is the explanation of why things fall down. They, like evolution, are two separate things.
When someone says "I don't believe in evolution" it's the same thing as saying "I don't believe in gravity".