This kind of music is scored for small group of instruments from two to ten instruments, which traditionally could fit a palace chamber. Mostly three to five movements.
Sometimes sonata is included to chamber music because it have one or two instruments but sometimes not because sonata provides only performance of solo instrument.
This kind of music is born since late Baroque, supposed that it originated from sonata in that period. Later, it was developed and went to the peak in Classical. (See Sonata in my blog)
String Quartet is an example of chamber music ensemble.
Ensembles
1. Duo or Duet - for two instruments. Example: Piano Duet.
2. Trio - for three instruments. Example: Piano Trio, consists of piano, violin and cello.
3. Quartet - for four. Example: String Quartet, the well-known ensemble. Consists of two violins, viola and cello.
4. Quintet - for five. Example: Piano Quintet. (See Recommended songs)
5. Sextet - for six.
6. Septet - for seven.
7. Octet - for eight.
8. Nonet - for nine.
9. Decet - for ten.
Recommended songs
Piano Quinter in A major, D. 667 by Schubert - known as Trout Quintet. This song is scored for violin, viola, cello and double bass. It got this name because there is a set of theme and variations in the fourth movement on Schubert's earlier Lied* "Die Forelle" (The Trout). Five movements in this song.
* A kind of vocal song, arranged for single singer and piano.