What is Barbershop? 
Barbershop harmony, as codified during the barbershop revival era (1940s-present), is a style of a cappella, or unaccompanied vocal music characterized by consonant four-part chords for every melody note in a predominantly homophonic texture. Each of the four parts has its own role: generally, the lead sings the melody, the tenor harmonizes above the melody, the bass sings the lowest harmonizing notes, and the baritone completes the chord, usually below the lead. The melody is not usually sung by the tenor or bass, except for an infrequent note or two to avoid awkward voice leading, in tags or codas, or when some appropriate embellishment can be created. Occasional brief passages may be sung by fewer than four voice parts.
According to the Barbershop Harmony Society,
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.
Slower barbershop songs, especially ballads, often eschew a continuous beat, and notes are often held (or sped up) ad libitum.
The voice parts in men's barbershop singing do not correspond closely to the correspondingly named voice parts in classical music. Barbershop singing is performed both by men's and women's groups; the elements of the barbershop style and the names of the voice parts are the same for both.