Imitation, Canon & Round Theory Things   William Wieland
The following two paragraphs are from Harold Owen's Modal and Tonal Counterpoint: from Josquin to Stravinsky. Schirmer: 1992.
  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 puzzle canon is one where the composer gives only the dux, or leading voice, and it is up to the performer to discover the correct time and interval distance for the comes, or following voice ( or voices when the canon is for three or more parts). Canon Worksheet

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: