The clavichord is of the piano family of string instruments, and has a large set of strings with a keyboard. The strings are hit by a small piece of metal (typically made of brass) called a tangen, and mounted at the end of a long stick-like extension of the key. A vibrato effect may be achieved by altering the finger weight on the key, which keeps the tangent in contact with the string.
The name is derived from the Latin clavis (which means key). The clavichord was probably invented in the 1200s, and was the main keyboard instrument for a long time - especially in Medieval, Renaissance, Baroque and early Classical music, until it was replaced by the more versatile piano.
Source: http://www.miayf.org/keyboard/clavichord/
http://www.youtube.com/watch?v=_GJwy-6x5hY