Technically speaking, barbershop harmony is a style of unaccompanied singing with three voices harmonizing to the melody. The lead usually sings the melody, with the tenor harmonizing above the lead. The bass sings the lowest harmonizing notes and the baritone provides in-between notes, either above or below the lead to make chords (specifically, dominant-type or "barbershop" sevenths) that give barbershop its distinctive, "full" sound.
Probably the most distinctive facet of barbershop harmony is the phenomenon known as expanded sound. It is created when the harmonics in the individually sung tones reinforce each other to produce audible overtones or undertones. Barbershoppers call this "ringing a chord." Singing in a quartet or chorus and creating that "fifth voice" is one of the most thrilling musical sensations you'll ever experience, leading to goosebumps the size of golf balls.