What is chamber music? The best definition is music for a small group of instruments. It originated in the 17th century when composers were hired by aristocrats to write music for them. It was played in smaller spaces by smaller groups or even one player.
More familiar forms are the string quartet, string quintet, String sextet and string octet. Also piano trio piano quartet and piano quintet. And the sonata for violin and piano and the sonata for cello and piano.
Most composers have written chamber music either a string quarter or a piano trio. many such as Beethoven, Mozart and Schubert wrote a lot of it.
As stated a chamber orchestra is a smaller of 50 or less players. Most of them do play chamber music. Some of them such as the Orpheus Chamber Orchestra do not have a conductor. They believe that chamber music does need a conductor or was intended for a conductor.
There are several chamber music societies in America and Canada : Chamber Music Americaacmp.net, the New York based Chamber Music Society of Lincoln Center chambermusicsociety.org and the Ottowa Chanber Music Society chamberfest.com to name a few.
There are many famous chamber ensembles besides chamber orchestras such as the Tokyo Quartet, the Cleveland Quartet to name a few. Violinists such as Itzak pearlmen, Isaac Stern and Yihudi Menuhen anad cellists such as Pablo Casals, Mstislav Rostropovich and Yoyo Ma have all played concertos and camber music.
There are many famous String Quartets and Quintets. There are 2 basic types of String Quintet: 2 violins, 2 cellos and 1 viola. The next type is 2 violins 2 violas and 1 cello. There is also a less common one for 2 violins,1 each cello and viola plus a double bass.