A chorus is a modulation effect that takes the initial audio signal, copies it, and then mixes the resultant sound with the original sound with some alterations in frequency.
A flanger is an audio effect produced by mixing two identical signals together; one signal delayed by a small and gradually changing period.
A phaser is an electronic sound processor used to filter a signal by creating a series of peaks and troughs in the frequency spectrum.
This lesson explains the chorus, flanger and phaser effect. These three effects are quite similar in the sense that they add a delayed copy of the original signal to the over sound to create their effect.