If you want to go DEEP into the science behind effects pedals, this page offers a pretty comprehensive breakdown of the difference between flangers and phasers, two very popular pedals.
The post includes lots of cool diagrams like this:
The technology of phase shifters and flangers
Practically every guitarist has heard the swirling, spacey sounds characteristic of a phaser or its cousin, the flanger. These boxes have a unique sound that is immediately recognizable to us in rock and processed acoustic music. Unfortunately these effects are not the easiest effects to understand in terms of how that odd tone is made. The terms “phase shift” and “flange” are not part of our daily lives, and don’t really convey up any mental image that’s useful in understanding them.
What is phase shift?
There is only a little electronics knowledge needed here.
For more visit: http://www.geofex.com/Article_Folders/phasers/phase.html