An effects processor adds something to the sound that recreates a sense of space or distorts the signal in some way for a special audio effect. Here are some common types of effects:
A reverberator or reverb unit creates sound that imitates the sonic reverberations of a room, the sounds that remain after an original sound stops. It comes in many flavors, imitating different size and reflective spaces, different kinds of reverb generation, optimized for drums or vocals, or artificially cut to produce snappy percussive sounds, for example. Typically, a multi-effects processor offers a preset selection of different reverbs and has a few controls that can modify each effect's tempo, decay, depth, or some other variable.
Reverb is non-uniform and chaotic, like natural room reverberation. A digital delay reproduces an exact duplicate of a sound and plays it back as an echo delayed by almost any time you set. A digital delay unit can then manipulate that echo to repeat with a gradual decay, for instance, or play back with a totally different tone. "Slap back" echo, a favorite in rockabilly recordings, consists of one or two very quick and crisp echoes, emulating the sound of a gymnasium, that impart excitement to up-tempo songs.
Another delay effect is called "chorusing" or "doubling." A chorus echo makes a single voice sound like a whole chorus of voices by delaying and slightly detuning a multitude of delayed signals. Doubling makes a soloist sound like a duet. Another kind of digital delay device is a "tapped" delay, which lets you "tap" a signal at multiple times after its initial attack for some very natural-sounding echo effects.
Harmonizing and pitch correction
Not only can you chorus a voice or instrument, you can create 3-part harmony, for instance, from a single voice with a "harmonizer." You can also add bass parts pitched below the original. These processors often also feature pitch correction, which can create an on-key version of a less-than-perfectly sung performance.
Modulation and distortion effects
And finally, there're effects processors that make unique and strange sounds that are completely unknown in nature. "Tremolo" or "Leslie" effects quickly and repeatedly raise and lower the pitch of a sound, giving it an unworldly, science fiction feel. "Flanging" is a distortion effect that seems to make a sound flare and swirl around in space. And "wah wah" aptly describes what such a device does to sound, like flapping a mute on and off in front of a trumpet.