Chorus is an electronic effect that attempts to make a single sound source sound like several sound sources playing together
In guitar terms it's an effect where instead of just the single note being played you hear additional notes as well. These notes share the same tonal qualities but are variations of pitch higher or lower than the original.
It gives the guitar the same effect a choir gives a single voice.
Noison.com has a great page showing carious chorus pedals and their sound effects. You can see that here: Chorus Effects
Chorus is often used on clean tone electric guitars to make the guitars sound larger or to give the effect of multiple guitars with a single instrument.
Chorus is often described as having a shimmering effect.
Original Author - Jer