Properly, repertory is a style of a number of repertory companies which rehearsed and performed plays in a fortnight. Originally a British idea, these were professionals but due to time restraints and commercial restraints they played like amateurs. The largest repertory theatre company was and still is in Liverpool.
There was a form of touring repertory called fit-up which involved carting round the set for about five different plays. The plays were shown on consecutive nights.
Nowadays repertories perform just once or twice a year.
The term is used in the theatre to refer to any number of two or more plays which are rotated within a season, usually alternating with different plays every night for a period of time. Plays are rehearsed at all once or in rapid succession, and often feature the same actors or company in several plays.
see also: Summer stock