Phaser

A phaser is a guitar effect that uses a copy of the original sound altered to be partially out of phase with the original signal to create dips in the frequency output when the two signals are remixed.

I'm certain there is more to the phaser - this stub is designed to encourage someone who knows better to improve this answer!

Phaser and Flanger

Both Phaser and Flanger effects are "Comb Filters".

A comb filter combines a wave with a very slightly delayed version of itself, causing both constructive and destructive interference. The result is that some frequencies are boosted, and some are cut. A graph of the frequency response has evenly spaced spikes, hence the "comb".

In a flanger, the delay in the signal is equal for all frequencies, and is slowly varied. This causes the accentuated frequency to sweep up and down.

In a phaser, the delay is achieved by an "all pass filter", which delays some frequencies more than others. This creates an other-worldly "wooshy" effect.