Speaking philosophically, free will means that a person's actions are determined by choice and not by a predetermined set of circumstances, a fate, or a higher power.
If you have free will, then you are in command of your behavior. If you don't have free will, then your actions are decided already, either by the circumstances of your biology, your society, or something else.
Free will is fundamental to our assumption of justice and law. If we believed, truly, that people were not in command of their own decisions, we would have to change the way we view right and wrong.
How could we blame Mr. X for stealing a car if he was predetermined to do it and had no power over that decision?
Science suggests that there is probably a bit of both (determination/behaviorism and free will) at work in all of us. We may be hard-wired to choose certain actions. We may be socially predisposed to act certain ways also. Yet, in the end, there is something in us (our conscience) that agrees or disagrees with an act before it is taken.
Certain disorders complicate this notion. But, mostly, that is how our culture views the motivational nature of man.