The harpsichord produces a sound by plucking the strings within its soundboard. The sound of a harpichord is therefore closer to that of a steel-stringed guitar rather than a piano. The harpsichord, an instrument which predates the piano, is particularly associated with music of the Baroque era when composers including JS Bach, Handel and Vivaldi were writing music. Although many composers wrote solo music for the harpsichord - perhaps the most famous being the sonatas of Scarlatti - one of its main functions was harmonic, providing a continuo within an ensemble. The dominance of the piano throughout thelast two-hundred years has diminished the role pf the harpsichord to a gerat extent. However, both contemporary composers looking for distinctive sounds within their music, and advocates of authentic performance of early music mean that the harpsichord has not been completely supplanted by its more ubiquitous cousin.
The organ, one of the oldest instruments in western music, uses a series of pipes to produce its distinctive sound. The timbre of these pipes is controlled by stops and the organist plays the notes using a keyboard plus a series of foot-pedals. Becuase the fingers and the feet have a role in playing notes (as opposed to the piano where the feet control other aspects such as dynamics) playing the organ requires very good coordination, particularly when playing contrapuntal music which simultaneous lines of music. Organs have a strong association with Christian music and the timbre of them is particularly suited to accompanying church choirs.
Developments in electronics throughout the twentieth century allowed for the creation of entirely synthesized sounds. The timbre of any instrument is distinct from that of another instrument because the different overtones it produces across the harmonic series when blown, struck, or bowed. By breaking down the sound of, say, a clarinet into its harmonics - the different rates at which the air column in the instrument vibrates - and mapping this across a series of electronic generators, one can approximate the sound of the clarinet electronically. This process, adding the layers of a sound together to create a whole, is called additive synthesis. This process is often expensive as it requires a number of generators and the final result is often only a rough approximation of the instrument. Other types of synthesis, which are much more complex, allow for the creation of much more accurate sounds.
Of course, the synthesizer can also be used to synthesize completely new sounds which bear little resemblance to orchestral instruments. The ability to alter sounds and change almost every component within them has lead to composers having an almost infinite palette of timbres available to them.