A synthesiser is an electronic musical instrument that produces sounds by generating and combining signals of different frequencies. Digital synthesis involves the use of a frequency synthesiser processor component which calculates mathematical functions, the results of which are signals of different frequencies.
The following is a very short overview of the basic categories in which synthesisers fall:
In the early days, each component of a synthesiser was self contained in its own individual box - a module. Hence the term "modular".
These synthesisers use analog based circuitry, such as oscillators and filters, that are controlled by voltage.
These are synthesisers that use digital methods to generate sound. Typically they start out with a sound source and then process it through various digital algorithms.
These digital synthesisers model 'organic' musical instruments with software instead of using hardware methods.
Instead of creating real acoustic sounds, synthesisers produce electrical signals which are then amplified through a loudspeaker or headphones. You use a synthesiser in much the same way that you would play a piano; via a keyboard. A synthesiser mimics the hammer striking a string in a piano by turning on and off electronic circuits.
Although the first synthesiser was built at RCA, USA in 1955, people had already started experimenting with producing music electronically as early as the 1870s. The first electronic instrument was invented by Leon Theremin in 1920 and was called the aetherphone (later renamed the theremin). The theremin has been used in places like the soundtrack to "Forbidden Planet" and teh Beach Boys' song "Good Vibrations".
The first synthesisers were very expensive and hard to play. We had to wait until the 1960s before synthesisers became much more user friendly. Pioneers on the synthesiser scene of that era include Don Buchla and Bob Moog (of legendary MiniMoog fame). By the 1970s, synthesiser production was on a much larger scale.