Behavior describes an individual's reaction to stimuli, primarily social stimuli. Unusual behavior can be an indication of mental illness, but can also be the result of physical illness. For example, Wilson's disease causes behavioral changes similar to schizophrenia, but must be treated differently.
Unlike personality, behavior can change relatively quickly, and is often molded for social situations. For example, House's personality doesn't change when he is around his parents, but his behavior does. However, sudden changes in behavior without external stimuli is often an indicator of mood swings, and can also be an indication of disease.
House is very sensitive to behavioral changes in the people he knows, generally putting their changed behavior down to some external stimuli. For example, in Que Será Será, he noted (with some respect) that Cameron's usual caring about the patient had escalated into risking hospital equipment and telling off Cuddy. He guessed that she was close to someone with a weight problem (like the patient) and even surmised that she had once been overweight herself.