Chamber music

Chamber music means music written for small groups of instruments. A "chamber" is a "room" (from the French word "chambre"). Usually the word "chamber" in English means a room in a large house or castle. In the days when people with big houses or castles kept their own musicians, they might have their own private orchestra which played in the large hall. Sometimes there would be a concert in a small chamber. This was called "chamber music".
Chamber music can be any group of instruments from two up to nine. Each player will be playing something different from the others ("one to a part"). Unlike an orchestra where there may be many violins all playing the same notes.
The word "chamber" is also used for a "chamber orchestra" meaning: a small orchestra. By contrast: a large orchestra is often called a "symphony orchestra". A small choir may be called a "chamber choir". But these examples are not usually thought of as "chamber music".
Words for the size of groups
These words are used to show how many people are playing. They can also be used for groups of singers (vocal solo, duet etc.):
a solo is just one player (or singer). This is not usually called chamber music.
a duet or duo is a piece for two instruments. The word "duo" usually means a piece in which the two parts are of equal importance. Violin sonatas, flute sonatas etc. usually have piano accompaniment but are not normally called "duets". Piano duets are for two players at one piano.
a trio is a piece for three instruments. Piano trios are pieces for piano, violin and cello. Sometimes trios may be called after one of the instruments, for example: Brahms' "Horn Trio" is for French horn, violin and piano, not for three horns!
a quartet is a piece for four instruments. String quartets are the most popular form of chamber music. They are for two violins, viola and cello. A piano quartet would be for piano, violin, viola and cello.
a quintet is a piece for five instruments. A string quintet can be for two violins, two violas and cello (e.g. Mozart's string quintets) or it can be for two violins, viola and two cellos (e.g. the one by Schubert). A piano quintet is for piano and string quartet (although Schubert's Trout Quintet is unusual: it is for piano, violin, viola, cello and double bass).
a sextet is a piece for six instruments. String sextets are normally for two violins, two violas and two cellos. Poulenc's Sextet is for piano and five wind instruments.
a septet is quite unusual and could be for any combination of instruments. Beethoven, Saint-Saens and Ravel all wrote septets, but for different combinations.
an octet is for eight instruments. Schubert's Octet is for clarinet, horn, bassoon, string quartet and double bass. Mendelssohn's Octet is for two string quartets.
a nonet is quite rare. Louis Spohr wrote a nonet for violin, viola, cello, double bass, flute, oboe, clarinet, bassoon and French horn.
History
Chamber music for instruments became popular as something different from big orchestras. The orchestra developed in the 17th century and so did chamber music. Composers wrote trio sonatas which were for two high instruments (e.g. two violins) and a continuo accompaniment (usually harpsichord and cello). Arcangelo Corelli and Johann Sebastian Bach wrote many trio sonatas.
Joseph Haydn wrote lots of string quartets. He made this combination popular. Mozart and Beethoven also wrote some very great string quartets. The last ones that Beethoven wrote were very difficult to play and to understand, but composers of the 19th century like Schubert, Schumann, Mendelssohn and Brahms were inspired by them. In the twentieth century some of the best string quartets were written by Bartok and Shostakovich.
Playing chamber music
It is great fun to play chamber music because each player is an individual. It is like having a conversation in music. There is no conductor, so each musician has to listen carefully to the others and learn to play together as a small team. The musicians can also hear their own playing better than they can in an orchestra. Some living composers have written a lot of music for young players who are not very advanced. Some of Mozart's earlier string quartets are not too difficult and make an excellent introduction to chamber music playing.