What is a cappella?
A cappella is not a genre but a style of musical performance: any music performed by voices alone (or, to be a bit more inclusive if a bit less precise, by the human voice, vocal apparatus, and sometimes body alone). Although any type of music may be performed a cappella -- and every type has been! --, thanks to the popularity of contemporary a cappella, the term is often used to mean voices-only performance of modern genres of music from the 20th and 21st centuries, including barbershop, doo-wop, hip-hop, jazz, pop, R&B, and rock and its derivatives.
Historically, the Italian term a cappella referred to music performed "in the style of the chapel": vocal music either without instrumental accompaniment or, at most, with instruments only duplicating the vocal line(s). As "chapel" implies, one strong tradition of such vocal music was non-secular or sacred in subject-matter; it was often liturgical, performed as part of religious service. Whether sacred or secular, the earliest attested vocal music was monophonic, consisting of a single vocal line or melody.
