Aristotle defines tragedy and comedy in the first book of the Poetics. He states that
"Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament ..." (Poetics I.vi)
while according to Aristotle,
"Comedy is ... an imitation of characters of a lower type - not, however, in the full sense of the word bad, the ludicrous being merely a subdivision of the ugly. It consists in some defect or ugliness which is not painful or destructive." (Poetics I.v)
Aristotle describes both tragedy and comedy as they existed in ancient Greece, as being performed by a chorus and no more than three actors (who were all male and wore masks), being written in verse, and possessing unities of time, place, and action. In general, tragedy dealt with people who were great and noble but suffered misfortune (often stemming from bad judgment), and comedy with people who were generally ludicrous rather than admirable.
Modern tragedies and comedies differ from ancient ones in being mainly in prose rather than verse, lacking a chorus, not having actors wear masks, having female actors normally playing female roles, and having no inherent limit on the number of actors. Thematically, although there are modern plays which are purely tragic or purely comic, far more common is a mixture of the two. Even Shakespeare mixes comic or rustic scenes, such as the gravedigger scene in Hamlet (V.i), into tragedies. Also, while some modern plays have unities of time, place, and action, others do not.