A mental disorder in which the ability to function is impaired by severely distorted beliefs, perceptions, and thought processes.

Positive symptoms of schizophrenia:
 

Excesses of normal functioning

• Delusions (false beliefs)
• Hallucinations (false perceptions - seeing or hearing things that aren't there)
• Disorganized thoughts and behavior

Deficits in normal functioning

• Low levels of motivation (avolition)
• Low levels of emotion (flat affect)
• Low levels of speech (alogia)

Links:
http://psychcentral.com/disorders/sx31.htm