In a canon, the theme, presented by one of the voices is imitated by the other voice or voices. A canon might be a complete musical work or be used as a counterpoint technique in a piece. Follows a very simple canon by Konrad Max Kunz:
There are many types of canons. The imitation of the first voice might be at the octave or at any other interval (2nd, 3rd, 4th, etc.). Besides, the imitation might use inversion, retrograde motion, augmentation or diminution.
See Canon for more information.