A play is a common literary form consisting chiefly of dialog between characters, usually intended for performance rather than reading (though many scholars study plays in this more solitary manner).
Plays are generally performed in a theatre, by actors. To better communicate a unified interpretation of the text in question, actors are directed by an aptly named director, who often puts his own spin on the story. For example, William Shakespeare's A Midsummer Night's Dream has been done with the underlying assumption that the entire story was in fact an acid-trip, which the bard likely did not have in mind when he authored the play.
The interpretive nature of drama is what makes it so appealing to so many performers and audience-members alike -- because a playwright is incapable of presenting the play in its intended format (a performance) without the aid of the actors and a director (though he may choose to take any of these roles himself -- Molière, for example, often acted in his own plays), a play is by definition undergoing constant rebirth and renewal as new experiences and interpretations are brought by new contributors.