The tendency for people to think that a person's positive or negative traits "spill over" from one area of life to another.
Halo Effects occur when one good quality about a person—for instance, a woman keeps a highly organized home—leads us to assume she is organized in other areas of her life. In a school context, a student who receives an "A" on the first piece of homework might end up getting undue slack from a teacher on further grading, because the teacher expects him to continue producing A-work. The inverse of the halo effect is the "Devil Effect" or the "Horns Effect," where one instance of bad performance causes the victim to be attributed negatively in an unfair fashion in the future.
The halo effect is a cognitive bias, a "mental shortcut" or even "cognitive illusion," that causes people to behave in ways that an unbiased observer considers unjustified. Because our entire lives are permeated by these cognitive judgments, they affect the very fabric of our society. So hang it there and keep learning them on this website. Metacognition rocks.