Choirs
Typically, a choir is led by a conductor, in much the same way that a conductor leads an orchestra. Singing in a choir isn't just a matter of opening your mouth and belting out tunes, either. The members of the choir have to be perfectly coordinated or the performance will sound “off,” although it may be difficult to pin down why the performance sounds wrong. A good choir is composed of people who sing together, perfectly on pitch, with voices and volume levels which complement each other.
Some communities have casual community choirs which welcome anyone who wishes to join. Typically people who want to join such choirs must demonstrate an ability to sing and read sheet music, and they must be able to commit to a practice schedule. Choirs can also be found in educational institutions, religious parishes, and collectives of musical performers such as city orchestras and operas.