Quite simply, energy is the ability to do work. And fuel is needed to create that energy. Let's look at a few sources.
Gasoline is what's called a nonrenewable source.
That means, there's only so much to go around. It's already in the earth as oil, and when it's used up, there is no more. Coal, oil, natural gas, even the uranium needed to make nuclear power are nonrenewable sources. The fuel a sailboat uses is wind. Wind is an example of a renewable source.