Have you checked a dictionary?
A stereotype is a depiction of a group of people, usually of negative attributes. There is usually some basis in fact, but it is not good to assume all members of a group are alike.
A prejudice is a pre-formed judgment on a person or thing. You make a judgment not based on the individual, but on his group. If you believe in stereotypes, then you are likely to form prejudiced opinions against an individual based on his group.