Delay, Chorus, Reverb, and Flange
Delay is a standard effect that many players use. It's a simple effect that takes your original guitar tone and duplicates it, sending it back through the amp at a later time. The effect gives you the feeling of playing in an echo chamber. If the delay is quick, it can be used to fatten up a guitar tone.
One of the most creative uses of delay is by U2's the Edge. The Edge uses delay in a unique way; he sets the delay very fast with many repeats and when he plucks one note, you hear four or five successive ones. Delay, when used, is not always noticeable, but it can have a dramatic effect on your sound.
Chorus is an effect that takes your original signal, makes a copy of it, slightly detunes it, and sends it to the amp mixed with your original signal. The effect sounds as if more than one player is playing, and the detuning is to approximate the sound of two players who are never perfectly in tune. Chorus is a standard effect on clean guitar sounds and can be applied to distorted sounds as well.
The market has recently seen the introduction of multi-effect floor units. These units usually contain reverb, distortion, delay, and chorus, and, like racks, can be configured in many custom setups. Many of these units have pedals to switch effects on and off. Most of all, these multi-effects floor units are convenient and easy to carry, unlike racks, which require heavy rack carriers, and unlike single pedals that eat batteries and require many small patch cables. Multi-effect floor units are made by Boss, Roland, Korg, Line6, Behringer, Digitech, and others.
Reverb tries to duplicate the natural echo of playing in a large hall, giving the effect of distance and aural space to your sound. Reverb is a subtle effect and can really improve your standard guitar tone; in spite of that, don't use too much of it because it can obscure your sound. Most amps come with onboard reverb, but some players like to have more control, so they opt for outboard gear.
Flange is an effect that sounds like a jet plane swooshing overhead, and duplicates the old recording trick of touching the flange head on a tape recording machine. Soundgarden used flange on “Black Hole Sun.”