A choir is a group of singers who perform together, with or without accompaniment from musical instruments. You may also hear a choir referred to as a chorus. Choirs around the world are incredibly varied, from casual secular groups which meet and perform periodically to highly organized church choirs featuring an array of very disciplined singers. Both secular and religious music is composed for the choir, and some choirs also adapt existing musical pieces for performance.
The history of the chorus is quite ancient. The Greeks used a chanting chorus to accompany stage performances, for example, and the medieval church also utilized chanted plainsong to accompany religious services. Gradually, the concept of a polyphonic or “many voiced” choir began to emerge, with musical compositions featuring multiple parts which could be sung together or individually, and modern choirs typically perform polyphonic compositions which showcase a range of voices.
There are many different types of choir. Mixed choirs feature both men and women, and it is also possible to find men's choirs, women's choirs, and children's choirs, including children's choirs divided by gender. The type of compositions performed varies considerably as well, depending on the members of the choir and the environment in which they perform. http://wiki.answers.com/Q/What_is_choir A choir, chorale or chorus is a musical ensemble of singers. Choral music, in turn, is the music written specifically for such an ensemble to perform.
A body of singers who perform together as a group is called a choir or chorus. The former term is very often applied to groups affiliated with a church (whether or not they actually occupy the choir) and the second to groups that perform in theatres or concert halls, but this distinction is far from rigid.
The term "Choir" has the secondary definition of a subset of an ensemble; thus one speaks of the "woodwind choir" of an orchestra, or different "choirs" of voices