What is chorus?
A chorus effect is produced when the sound from your guitar is mixed with one or more delayed and slightly pitch-modulated copies of itself. Your signal goes into the chorus pedal which blends your tone with the altered copies produced inside the pedal. The sound can be achieved natural by doubling tracks, something that has been done in the studio by people like Randy Rhoads and Johnny Marr. Yo can also hear the chorus effect when listening to a choir with many people singing each part, or in a string orchestra where each part is played by more than one person. The sound that´s produced can be described as "ambient", "dreamy" or like a big "shimmer".
Any good examples?
There are many classic examples of chorus on guitar songs, and very different ones. For example, Kurt Cobain´s sound on "Come As You Are" differs a lot from The Pretenders "Mystery Achievement", and James Hetfield´s tone on "Welcome Home (Sanitarium)" is far from the guitar in John Scofield´s tune "Just My Luck". Chorus is an effect that is used in many different styles, from Heavy Metal to Jazz, from Pop to Funk.
Which pedal should I get?
It all comes down to taste but a great pedal for those classic Police-tones and more is the Boss CE-2. The obvious choice for the Nirvana fan is the Electro Harmonix Small Clone, which has a clever depth switch. The MXR M-134 Stereo offers a wide range of options and the TC Electronic Corona Chorus also sounds really good. However, many jazz players (and others as well, like James Hetfield) prefers getting their chorus straight from the amp, not unusually a Roland JC-120 Jazz Chorus Amp. Try some of these out and find your favourite!