What do they do?
Chorus, flanger and phaser effects are all considered modulation effects. Flanging sounds as if the frequency spectrum is sweeping up and down.
The Small Faces’ Itchycoo Park contains a classic example of the sound, though it has been heard in many other places, too.
Chorus effects do just what you think - they simulate multiple copies of the input signal playing simultaneously, though each with a slightly different pitch and time. Nirvana’s Come As You Are is rife with chorused guitars.
Phasers sound similar to flangers, but there is a sweeping ‘notched’ sound in play, giving them their characteristically metallic, piercing tone. Think Jean Michelle Jarre’s Oxygene here.
How do they work?
Each of the above effects makes use of copies of the incoming signal. A phaser alters the phase of the copy with one or more all-pass filters. The number of filters is defined as the number of ‘stages’, and will affect the depth and richness of the sound.
A flanger is very similar, except that the copied signal is delayed by a few milliseconds. This results in a sort of ‘jet engine’ sound. Similarly, a chorus makes use of at least one copy of the original signal and plays it back at a slightly different pitch, and usually with some amount of modulation as well.
Use them on...
The most important thing to remember with these effects is that they are exactly that: effects. Though in some cases (most notably the chorus effect) they attempt to emulate real-world techniques, they sound artificial and synthetic. They are ideal for sweeping guitars, electric pianos and even the occasional bass track.