Chorus works by playing a copy of the sound coming in again just slightly later than the original (like 100ms later) and lightly altering it's pitch. This makes it sound like 2 or more instruments are playing the same thing at the same time, a chorus of instruments if you will. The Boss CH-1 is a nice simple chorus pedal.
A flanger works very much like a chorus, except that the delay is much shorter and a portion of the delayed signal is fed back into the delay. This creates a swooshing sound. The term flange actually comes from when in the reel to reel studio days they would have 2 synced up tape machines and press their finger on the 'flange' of the tape reel, slowing it down a little and the combined sound would give the same effect. This MXR Flanger pedal seems to be pretty popular although I have not played it myself.
A phaser although it has a similar sound to a flanger works in a different way. The sound rather than being delayed has it's phase shifted and added back to itself based on a filter. This phase shifting creates the phaser sound
What Do I Use?
Right now the only one of these 3 pedals that I use is a Boss CH-1 Chorus pedal. I personally have never been a huge fan of flange/phase effects so I have not spent any money on them. But that is just a personal preference as there is certainly nothing 'wrong' with those effects. They can be used very nicely, but please, use in moderation :)
Do you have a modulation effect that you love? If so sound off below and let us all know what you have discovered!