A musical ensemble is a group of several musicians who gather to perform music. There are several denominations of ensembles according with their size and composition.
The terms duet, trio, quartet, quintet, sextet, septet, octet, and nonet are used to describe groups of two, three, four, five, six, seven, eight, and nine musicians, respectively. A common quartet is the string quartet, composed of two violins, a viola and a violoncello. The most usual string quintet is similar to the string quartet, but with the viola duplicated. In some cases, though, it is the violoncello that is duplicated. A piano quintet is usually a string quartet plus a piano. Another fairly common grouping in classical music is the wind quintet, usually consisting of flute, oboe, clarinet, bassoon and horn.
In jazz, a fairly standard trio line up would consist of a rhythm section of piano, bass and drums. A quartet would typically add a horn (the generic jazz name for saxophones and trumpets) while larger ensembles would add further instruments. The actual lineup of jazz ensembles can vary quite considerably though.
A group with more instruments is usually called an orchestra. A small orchestra is called a chamber orchestra. A symphony orchestra is a large body of several tens and often more that a hundred musicians, divided in groups of instruments: violins (I and II), violas, violoncelloss, basseses, woodwinds, brass, percussion, and sometimes more.There is no difference between Symphonic, Philharmonic, and similarly titled orchestras. These are only names used to distinguish different symphony orchestras. A Sinfonietta usually denotes a somewhat smaller orchestra (though still not a chamber orchestra), and the terms concert or pops orchestra usually mean an orchestra concentrating mainly on the light classical and more popular repertoire.
A choir is a group of voices. Sometimes the group of similar instruments in an orchestra are referred to as a choir. For example, the woodwind instruments of a symphony orchestra could be called the woodwind choir.