"Fahren" is used in sentences like:

    Der Mann fährt ein Auto.

    Ein Auto fährt auf der Straße.

    Das Auto fährt in der Stadt.

"Befahren" on the other hand is used like this:

    Die Straßen sind befahren.

If you are into grammer, you can remember it like this: with "fahren" you can use a subject and/or an object - this means it is important to say who is the doer, what is done, how it is done, etc. With "befahren" you can use only a subject - here you say what type of road is used. This can be a streets, a sea route, etc.