Harpsichords are keyboard instruments that make sound by having the strings plucked with a plectrum. They are thought to have originated when a keyboard was attached to a psaltery. This makes them different from a clavichord where the strings are hit. The piano is therefore closer to a clavichord than a harpsichord. On a piano it is possible to play louder or quieter by playing the keys with more or less force. On a harpsichord the volume (playing louder or softer) cannot be controlled by the way it is played. However, some large harpsichords have several "stops", each one giving a different kind of sound. The largest harpsichords have two manuals (keyboards) which gives more variety. It makes it possible for the right hand to play the tune on one manual while the left hand plays an accompaniment more quietly on the other.
Some small harpsichords were called "virginals", perhaps because they were played by young girls. There are also spinets which were very small and sometimes wing-shaped. They could be picked up and put on a table.
Harpsichords were very important in music from the Renaissance and Baroque periods. They were used as solo instruments as well as to accompany an orchestra. Some of the most famous composers of harpsichord music are William Byrd (1543-1623), François Couperin (1668-1733), Domenico Scarlatti (1685-1757) and Johann Sebastian Bach (1685-1750). The last of these composers wrote The Well-Tempered Clavier, which has a prelude and fugue in every major and minor key. It is said that Bach wrote this collection of music to show how keyboard instruments like the harpsichord can be tuned in a way to make playing in any key possible.
Media related to Harpsichords at Wikimedia Commons