Short answers, pulled from the story.
A harpsichord makes sound by plucking strings. Depressing a key raises its back end, which lifts a jack holding a small plectrum of quill or plastic, and that plectrum plucks a single string. The strings vibrate over a soundboard that amplifies them so listeners can hear them.
In a harpsichord the strings are plucked, so a note sounds equally loud no matter how hard the key is pressed. In a piano, first built by Bartolomeo Cristofori around the year 1700, the strings are struck with hammers, letting the player control the volume of each note. That expressive control led the piano to largely supplant the harpsichord by the late 18th century.
Choirs are the additional sets of strings on harpsichords that have more than one string per note. They let the player vary both volume and tonal quality, and they are described in pipe-organ terms: eight-foot strings sound at normal pitch, four-foot strings an octave higher, sixteen-foot an octave lower, and the rare two-foot two octaves higher. The full set of choirs in an instrument is called its disposition.
Major harpsichord composers include William Byrd of the English virginal school, Francois Couperin, Domenico Scarlatti with his 555 sonatas, George Frideric Handel, and J. S. Bach, whose works include The Well-Tempered Clavier and the Goldberg Variations. Joseph Haydn and Wolfgang Amadeus Mozart also wrote harpsichord music early in their careers. In the 20th-century revival, Francis Poulenc, Manuel de Falla, and Elliott Carter composed for it.
The harpsichord family includes the virginal, a small rectangular instrument with one string per note, and the spinet, with strings set at about a 30-degree angle. Other variants are the clavicytherium with vertical strings, the ottavino at four-foot pitch, the pedal harpsichord with a foot-operated keyboard, and the folding harpsichord built for travel.
The harpsichord almost disappeared for most of the 19th century after the piano supplanted it, surviving mainly in opera to accompany recitative. It revived in the 20th century through performers like Violet Gordon-Woodhouse and Wanda Landowska under the influence of Arnold Dolmetsch. Mid-century builders such as Frank Hubbard, William Dowd, and Martin Skowroneck restored Baroque-era building traditions that dominate the scene today.