Brainwashing is the act of coercively convincing someone of misinformation (well -- usually misinformation). The victim becomes absolutely convinced of the new information.
On the subtle scale, brainwashing can be induced by the repetition of misleading or inappropriate information:
While the adult thinks it is a joke, perhaps, you can't tell a child over-and-over that they are ugly when a fact of life is the best jokes have a kernel of truth to them. Eventually the child will wonder if they really ARE ugly and as the torment continues, they will come to believe it.
The same goes for telling a child they are stupid, even if only a momentary lapse of reason in the face of an emergency: "How could you do something so stupid! What are you? An idiot?!"
In its most extreme forms, however, brainwashing is no accident. The deliberate feeding of misinformation to the victim is a calculated attempt to change their mind or even completely alter their reality.
Brainwashing can be difficult to overcome. Even though no one is telling me that "I am ugly and my mother dresses me funny." anymore, I can't look in a mirror and see myself the way other people do. Enough people have told me that I am pretty that I am sure they are not lying -- and some people in my head do not have the problem of image distortion that others in here do -- but this lie is firmly stuck in my psyche.