Serious and generally sparse music compositions that are meant to be played for a small audience in a small room or small hall. Usually played by a Quintet, Quartet or Trio of musicians with each playing a different instrument.
The word "chamber" in the name comes because it was, as was said before, mean to be played for a small audience in a small area and often times in a private room, and in the days it was popular (from the late 1600's to the late 1700's) a small room was also known as a 'chamber'. It was also referred to as "private music" in it's day.
Chamber music is the immediate predecessor to Baroque Music and successor to the more unstructured styles of the lone minstrel or religious music. Indeed it pull much from bard culture in that it is made to that the individual musicians can work on concert with each other and have it still sound good without much prior practice because each person is playing separate parts instead of playing in harmonies with one another.
Going on this, the basic structure of chamber music is many different melody lines that intertwine without being the same notes. A typical chamber Quartet would probably contain a violin, viola, cello and, if they had the means, a harp or harpsichord. Later in it's popular life flutes may well be included in an ensemble as well, thought generally they were restricted to string instruments.
Each piece in a 'concert' was generally presented as a package, as in "Here is a song, and here's another, and another" in contrast to the later classical style of the baroque era and beyond "Here is a song that kind of almost dies out, but wait it's gotten much different now and it's a completely different song!"
Well know composers of chamber music include Haydn, Müller and Sarasate among other composers who were not primarily composers of chamber music like Beethoven, Brahms and Stravinsky.