Audio effects for Guitars and bass guitars usually take the form either of pedals or rack mounted units. Pedals are by far the commonest.
Unlike guitarists many bassist do not use effects apart from any that might be built into amplifiers however there are many units that can be used with bass guitar.

The Effect Pedal

Most effects pedals have an input and an output which take the form of 1/4 inch jack sockets, the same type found on most electric guitars and basses. They have a foot switch to turn them on and off and some control knobs to adjust settings. There may be two outputs if the effect creates a stereo sound or can be used to send different sounds to two amplifier and there maybe multiple foot switches if more than one element can be switched on and off. The whole point of an effects ‘pedal’ is that it can be turned on and off while carrying on playing.

Types of Effect

Dynamics processors work by adjusting the volume of the sound. Examples are compressors which make quiet sounds louder and limiters which stop the volume exceeding a set level. Distortion units a related to limiters but they impact the sound in a more radical way, literally chopping the sound off at a certain level creating the familiar distorted sound.
Time effects work by creating copies of the signal and playing them back in different ways, reverb, delays, chorus and flangers work this way. The more extreme effects are gained by speeding sound up or slowing it down. Loopers work by recording a portion of sound and playing it back in a loop so that you can play over the top of it.
Pitch manipulation involves taking the sound and adjusting the frequency and then mixing in the original signal. Harmonisers and Octavers are examples.
Filters adjust the value of specific bands of frequencies and include graphic equalisers and ‘Wah’ pedals.