The phrase "a cappella" comes from the Italian for "in the style of the chapel."
In the past, "a cappella" referred to vocal music, originally church music, unaccompanied by instruments. That meaning is still in use, but another has been added.
Today, "a cappella" also refers to small vocal ensembles. Such groups have been around for a long time--the Yale Whiffenpoofs were founded in 1909--but their popularity has exploded in recent decades.
One characteristic of today's a cappella groups is the use of the voice to imitate instruments to provide a percussive accompaniment to the melody and tight vocal harmonies.
The popularity of a cappella is strongest on college campuses, with Harvard and Yale each boasting close to 20 a cappella groups. Many members stick with it after graduation, joining existing groups or forming new ones.