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