A canon is a type of musical structure in which a melody is begun and repeated by different voices or instruments at staggered intervals.
Among the several forms of canons are the strict canon, in which an exact imitation follows the start of the initial melody closely in terms of both intervals and rhythm, the free canon, in which the following melody may have some variations in rhythm and or intervals, the retrograde canon, in which the following melody is the reverse of the original melody, and the accompanied canon, in which the successive melodies are accompanied by a bass line or other harmonic support.
A round is a specific type of canon in which each voice or instrument begins singing or playing the same melody at a different time. Once a voice completes the melody, it can restart it, thereby producing a continuous cycle and allowing the piece to continue on indefinitely. Rounds are typically less complex than other forms of canons, including lacking in variations such as in intervals and rhythmic patterns. In canons, while different voices play of sing the same melody beginning at different times, they do not necessarily repeat the melody in a strict cyclical manner as in a round.