String instruments have one or more strings stretched across the hollow body of the instrument. When the strings are made to vibrate (by bowing or plucking, usually) the sound is amplified by the body of the instrument.
In an orchestra, there are four instruments in the string family. The two smallest, the violin and the viola, are held on the shoulder. The cello rests on the floor and is held between the player’s knees. The double bass is so big that the player must either stand, or sit on a high stool to play it.
The violin, viola, cello, and double bass are played with a bow. The strings of a bow are made from horsehair – the hair on the tail of a horse!