Although electricity has been used to make music since the 18th century, it did not become practical or economical until well into the 20th century. Electronic instruments are different from other instruments in that sound is not produced mechanically, but with electricity.
The vacuum tube, invented in 1906, and the oscillator, perfected in 1915, were crucial in the development of electronic music. Most electronic instruments use an oscillator to produce a pitch, and can raise or lower the pitch by increasing or decreasing the voltage, or power supply, flowing to the oscillator. By adding overtones and otherwise modifying the sound, the synthesizer is able to produce a wide variety of sounds and timbres.
It wasn't until the 1950's or 1960's however, that economical and easy to use synthesizers were created. The addition of a piano-like keyboard further improved control of synthesizers. Other control devices have also been created allowing musicians to control synthesizers as if they were guitars, drums, or even wind instruments.