Prejudice : When we judge something or have an idea about something before we actually know about them is prejudice, it is unfair reaction to something or someone.
Stereotype : It is generalisation or assumption people make about a group of people. For example we have heard people saying that only men can join army or only girls are cry baby. which is not true.
Marginalizing : When we treat group of people as insignificant or less important or inferior to us. For example we treat dalits improperly so we marginalize them in many ways.