This is a very intelligent question. It is probably good to start with a definition of these terms.
- A stereotype is the popular belief about someone, group, or thing with little basis in reality. It is to make gross generalizations.
- A halo effect is when one good quality of a person is used to make good generalizations about that person with no basis.
- Projection is when a person takes usually unwanted feelings or ideas and ascribes them onto others, when they are really from that person.
An example of stereotyping is when someone ignorantly says that all women cannot drive. This is completely false, not to mention chauvinistic and foolish.
An example of a halo effect is when we think that a celebrity whom we like, such as Tom Cruise, and assume that he is intelligent, kind, compassionate, and caring. He might be these things, but we cannot automatically assume them, just because we like his acting.
An example of projection is when a person says that she does not like me, when in fact that person is the one that does not like that other person.