When the parts in a piece of polyphonic music share melodic ideas, they are said to be in imitation. If the imitation is exact and continues through the whole piece, the piece is called a canon. A portion of a piece is described as canonic if the imitation persists throughout the section. In describing points of imitation there are two crucial measurements to consider: (a) the distance between leader and follower in time; and (b) the interval distance between the first note of the leader and that of the follower. Sixteenth-century canons often use interval distances of a perfect fourth or fifth as well as octave and unison.
A round is a familiar type of canon where the time distance is the length of a complete phrase and the interval distance is a unison (or an octave when men and women sing together). Rounds are cyclic: as each voice reaches the end, it begins again. A round is normally sung as many times through as there are voice parts.
Examples:
Source: https://www3.northern.edu/wieland/theory/parts/imitate.htm