A contrabass is a large European stringed instrument that belongs to the viol family. It produces deep (and very low) notes with a rich sound. It's lowest note is the E string (although this can be lowered even further with the help of a C extension or an additional string) which is the equivilant of the E two octaves below middle C on piano. The contrbass is common (although not nearly as common as a cello or violin) in jazz or classical pieces, although does not go very far in many other genres. Generally, the contrabass plays the harmony and is rarely given solos.
Contrabass belongs to the string family and reads in the bass clef.