What is 'a cappella' music?

'A Cappella' refers to vocal music which is performed without instrumental accompaniment. The term 'a cappella' hearkens back to a time when the music of the churches consisted purely of voices singing together; thus 'a cappella' simply means 'as in the chapel' or 'in the manner of the church'.

Nowadays 'A Cappella' takes many forms including unison plainchant, Perotin's famous polyphonic writing, the close harmonies of Barbershop, Rounds, Madrigals, straightforward four part harmony (as in hymns), cultural music such as Georgian folksongs and tribal music, and 'A Cappella Genre' (which usually consists of arrangements of popular or known music for voice and commonly features close harmonies, beat-boxing and vocal mimicry to replicate either instrumental sounds or to create a texture over which a solo line might be laid).

For examples of several different forms of 'A Cappella' music, search on youtube for groups such as Sons of Pitches, The Swingle Singers, The Sixteen, et c.