Because racism is defined as a system of advantages based on race, whites are the only ones who can be racist simply because there are no systems that minorities can benefit from by being racist.
Society has influenced many people to have racist behavior simply because that is the way that many people have been brought up. Its the idea that your enviornment effects your personality and morals.
Because racism defined as a system of advantages based on race, all whites are someway or some how benefitting in systems throughout their life.
Different whites benefit from racism differently because they all are involved and depend on different systems from one another. One system may or may not effect somebody as much as another system might.
Because racism is meant to provide an advantage to whites, some racism is not targeted to certain people of color. This occurs when these people of color are not involved within the certain system in which whites are trying to gain an advantage over.