The term 'chamber' comes from the word for a room as it is music to be played in a room rather than a large concert hall.
Although some chamber music was intended for amateurs to play at home, it is now often performed in a concert hall.
It is played without a conductor and uses one player per part (in orchestral music there are sometimes several players to a part).
Some of the more common chamber music combinations are duets (two players), trios (three), quartets (four) and quintets (five).
The trio sonata was very popular during the Baroque period (roughly 1600–1750). Many Baroque trio sonatas were written for two violins (or recorders, flute or oboe) plus continuo.
The continuo part was played by harpsichord (filling in the harmonies) sometimes with the cello playing the bassline - so there were often four players, not three. The harpsichord is a keyboard instrument where the strings are plucked rather than hammered.