In music, a capella is singing without accompaniment, or a piece intended to be performed in this way (see vocal music).
In recent popular construction of one a capella group(especially in Japan) is made up of Vocal, Chorus, and Bass. And sometimes voice percussion (or human beatbox) is added to enthuse the field of the live.
Vocal part is the one who sing the word in the songs and be as the center of the group. When the group is going to sing at like street concert, Vocal does not only sing as hard as and as well as they can, but also sometimes does some performance for watcher and make the field exciting.
Chorus or choirs are normally made up of three or four groups and make music with their voices. When they make music, most of the cases they use chord to make harmony.
And Bass .................