"Barbershop music features songs with understandable lyrics and easily singable melodies, whose tones clearly define a tonal center and imply major and minor chords and barbershop (dominant and secondary dominant) seventh chords that resolve primarily around the circle of fifths, while making frequent use of other resolutions. Barbershop music also features a balanced and symmetrical form, and a standard meter. The basic song and its harmonization are embellished by the arranger to provide appropriate support of the song's theme and to close the song effectively.
Barbershop singers adjust pitches to achieve perfectly tuned chords in just intonation while remaining true to the established tonal center. Artistic singing in the barbershop style exhibits a fullness or expansion of sound, precise intonation, a high degree of vocal skill, and a high level of unity and consistency within the ensemble. Ideally, these elements are natural, unmanufactured, and free from apparent effort.
The presentation of barbershop music uses appropriate musical and visual methods to convey the theme of the song and provide the audience with an emotionally satisfying and entertaining experience. The musical and visual delivery is from the heart, believable, and sensitive to the song and its arrangement throughout. The most stylistic presentation artistically melds together the musical and visual aspects to create and sustain the illusions suggested by the music."
Let's see some examples found on the Youtube:
Harmony during the 2005 International Chorus Contest
Westminster - Barbershop Chorus at the FWD contest