A guitar pedal is the name given to a self-contained device designed to
transform the output from a
guitar before going to an
amplifier.
Also known as an
effects pedal or
stomp box.
Always requires a power source (in my experience), usually a square 9-volt battery or a
transformer.
Typical effects would include:
...and many,
MANY more!
Also related is a
pedal board which contans many different effects in a larger board like structure, as opposed to the guitar pedals small box-like shape which can only (usually) do one effect.