What is a Synthesizer?

A synthesizer is an electronic musical instrument. The difference between electronic instruments and acoustic instruments can be illustrated with a few examples: a flute creates its sound by setting a pipe of air into resonation; a violin makes a sound as the player "bows" a string into self-oscillation, which is then transferred and amplified by a specially crafted wooden "case". Acoustic instruments are all mechanical - something must be moving or resonating for sounds to be created.

Synthesizers are different: they don't create direct sounds, but only electric signals. Without loudspeakers or headphones attached to the synthesizer, it will not be able to make anything even remotely audible - well at least nothing apart from the clicking of the keys. This is really not very far from how your CD-player works: it doesn't make direct sounds either, only electric signals that are turned into sounds by your stereo speakers or your headphones.

Synthesizers are usually equipped with a piano style keyboard. Each key of the keyboard is actually a switch by which the user can switch electronic circuits on and off. Keyboards are by far the most popular input devices, but the user can also choose to use mouthpieces, strings, guitar-like devices, drum pads or a computer to control the synthesizer.

Synthesizers are extremely versatile instruments. They can be made to imitate any other instrument - from reed instruments to drums. But the true power of the synthesizer is its possibility to create completely new, yet unheard sounds - even if not all of these sounds are useful for musical purposes.

References

http://www.synthzone.com