Effects units are devices that affect the sound of an electric instrument or other audio source (such as recorded material) when plugged in to the electrical signal path the instrument or source sends, most often an electric guitar or bass guitar. They can also be used on other instruments or sound sources, like the Rhodes piano, synths or even the human voice. While some effect units transform the sound completely, others just color the sound picture in a minor way.

An effects unit consists of one or more electronic devices which typically contain analog circuitry for processing audio signals, similar to that found in music synthesizers, for example active and passive filters, envelope followers, voltage-controlled oscillators, or digital delays.

Effects units are packaged by their manufacturers, and used by musicians, in various sizes, the most common of which are the stomp-box and the rack-mount unit. A "Stomp box" is a metal box, containing the circuitry, which is placed on the floor in front of the musician and connected in line with, say, the guitar cord. The box is typically controlled by one or more foot-pedal on-off switches and typically contains only one or two effects. A second type of effects unit may contain the identical electronic circuit, but is mounted in a standard 19" equipment rack. Usually, however, rack-mount effects units contain several different types of effects. They are typically controlled by knobs or switches on the front panel, and often by a MIDI digital control interface. "Off-boards" are used by musicians who prefer multiple stomp-boxes; these may be simply pieces of plywood with several stomp-box units fastened to the plywood and connected in series. Rackmounted effects or off-boards can combine several effects in one unit, and can include analog controls such as pedals or knobs.

Modern desktop and notebook computers often have sound processing capabilities that rival commercially available effects boxes. Some can process sound through VST or similar plugins, such as RTAS or Direct X. With a decent sound card, musicians can play any instrument through a computer, emulating any effects unit or even an amplifier in a convincing way. Many VST-plugins are freely downloadable from the Internet.