Effects pedals are a portable accessory that can be used with all instruments either by direct in-line connectivity or by connecting the pedal or processor to an effects send channel of a mixer or recording device. The processor is the brain power of the device that operates the software and by linking to the components - processes that particular effect or sound required.
Effects pedals are commonly floor mounted and therefore foot operated - which is why they are also frequently called foot pedals. Very often they will take the form of a small rectangular die-cast metal box with 2 to 3 Rotary controls on the upper surface for changing sound and effects parameters.
The effect is usually operated by pressing the foot on a hinged electronic switch located on the upper face of the pedal such that when the switch is depressed the effect is heard. This is also commonly joined by the illumination of an "active" LED so that a visual indication exists that the effect is turned on.
Effects pedals and processors can be for a specific singular effect such as distortion or reverb, alternatively they may be part of a multi-processor configuration where they are used in conjunction with other effects. They are connected to a sound source such as an amplifier or combo via a couple of guitar instrument cables that use ¼" jack interfaces usually labelled "instrument" and "amplifier" respectively.