Physicalism is the position that everything that exists does so within the limits of its physical properties and that there are no other kinds of things other than physical. It differs from materialism in that physicalism recognizes the existence of things (concepts, laws, etc.) that cannot be reduced to only matter. But, physicalism would maintain that mental states are identical to physical ones. So, physicalism would maintain that the laws of physics can't completely explain the entire universe and anything in it.
Like materialism, physicalism denies the existence of God and also leads to a denial of free will since it would mean that the human brain operates only under limitations of physical laws. These physical laws, neurochemical reactions, would determine what the person says in response to a stimulus.