Electronic music, at its core, is music produced primarily by synthesizing waveforms representing the tones of the music, as opposed to music produced by directly recording acoustic sources such as instruments and voices. Electronic sources such as synthesizers began to be introduced into all genres of music beginning in the late '50s and early '60s, and virtually all popular music includes at least some synthetic sources. However, the class of genres typically called "electronic music" is identified by the primary or even sole use of either synthesized sounds or short recorded samples, generated or played back in an automated, controlled way by a computer program called a sequencer to create harmonic music.
The primary subclass of electronic music is electronic dance music, composed for and primarily performed in nightclubs and similar venues. This overarching label encompasses several more specific genres such as house, techno, trance, and downtempo, each of which has even more specific sub-genres such as dirty house, deep house, techno-pop, industrial, trance-pop, heavy trance, ambient, chill, etc. Tracks generally falling into all of these genres have reached various levels of fame and popularity on music charts, and songs directly from or based on these genres now comprise a significant percentage of "Top 40" hits in the last two decades.
The music is composed primarily by using electronic keyboards and other input devices that interface with a computer using the Music Instrument Digital Interface or MIDI, which assigns each sound a start and end point on the timeline of the sequence, an instrument (typically assigned a "track number"), a frequency or note value to be played, and a "velocity" determining the volume and other characteristics of the tone to be played. All of these values can be further edited within the computer, and then played back by the computer itself or any equipment connected to it that has a MIDI-capable synthesizer (virtually all electronic keyboards, drum pads and other electronic instruments support this interface).