A stereotype is nothing more than prejudging someone before getting to know that person. We all prejudge to a degree. It could be as simple as seeing a blond and thinking they're ditzy when in fact they're very intelligent, to seeing an overweight person and thinking they're lazy when in fact it could be a medical condition. They become harmful when someone applies a negative stereotype to a group of people based on the actions of a few, ie...all blacks are criminals.
Discrimination on the other hand is the unfair treatment of a person or racial group based on prejudiced. An example would be a hiring manager setting aside a resume because the persons name is Kareem Abdul Muhammad, even though they are qualified for the position. The hiring manager may opt to interview someone named Gary Anderson just because the name sounds more American.