Chamber music is music written for small groups of instruments, for performance in a small room or 'chamber' from French 'chambre' = room. If performed in front of a small audience, in the old days the room would have been a grandiose aristocratic 'salon', but today the room is more likely to be a small hall.
A very important type of chamber music is the string quartet, written for 2 violins, a viola and a cello. The classical composers Mozart, Haydn, and Beethoven wrote some of their finest and most intense music in this category. The structure of a string quartet is usually in sonata form. The players usually form permanent quartets, because they need to play together all the time in order to excel in performance. No one instrument dominates the others, all are equal, and the players must listen to each other very carefully.
Other groupings of players for chamber music include - Piano Trios (piano, violin and cello), string trios (violin, viola and cello), piano quartets (= piano + string trio), piano quintets (= piano + string quartet), clarinet quintets (= clarinet + string quartet), and the wind quintet (=flute, oboe, clarinet, French horn and bassoon).