Here's a link from one popular song 20+ years ago. http://www.youtube.com/watch?v=vabnZ9-ex7o
Here's a good explanation, along with some more information on uses in popular music - HERE
While modified four-stage phasers like the Uni-Vibe and other effects before it had sought to evoke a choral-like sound, the chorus pedal as we know it didn’t arrive until affordable short-delay chips became widely available in the late 1970s. This in itself implies that chorus is a type of delay, and indeed it is, but the sonic results of the ways in which these short delays are transformed lands the effect firmly in the modulation camp, so it’s easiest to consider it as such (the function of these delay chips will be explained in more detail in the Echo & Delay section down the road). IC-based chorus effects work in a manner very similar to flangers, detailed above. The main difference, put simply, is that the short delays used to create harmonically spaced notches and peaks in the frequency spectrum are manipulated to modulate more tightly above and below specific frequency ranges, rather than shifted to swoosh broadly up and down the entire spectrum. The result, and intention, is a sound like a group of singers or similar instruments, where some voices/strings inevitably waver slightly from those beside them, producing a quivering clash of harmonics that evokes space and dimension when used subtly. The effect is possibly best heard—as pure effect, at least—in stereo, and the nature of the beast lends itself readily to stereo outputs. The broad soundstage and Doppler-like movement a good stereo chorus can spread out before you between a pair of spaced amps or L-R headphones make it one of the most spacious, three-dimensional effects available, short of long delays or cavernous reverb settings.
Boss’ CE-1 Chorus Ensemble was the first of these types commercially available, and is the best remembered of the company’s now-archaic looking early range of die-cast metal pedals. The unit was an instant success when it Boss CE-1hit the market in 1976, and was quickly snatched up by a range of major players. Andy Summers used the CE-1 with the Police in the late 1970s and early 1980s, and it’s probably most famously heard on the band’s big 1979 hit “Message In A Bottle,” though others made creative use of it too. Shortly after the Boss, Electro-Harmonix offered both its Memory Man Stereo Echo/Chorus—which featured a very good, spacious chorus setting that a lot of player’s loved—and smaller, stand-alone Small Clone chorus. Like the Small Stone phaser before it, the Small Clone had a softer, subtler sound than many of the chorus pedals that would soon flood the market, and it too was a huge hit. Kurt Cobain’s use of the pedal on “Smells Like Teen Spirit” and Chris Novoselic’s bass part on “Come As You Are” from Nirvana’s Nevermind album shows off how it can add a rich, moving, liquid texture to both clean and distorted parts. MXR, DOD and Ibanez all offered popular early IC-based analog chorus pedals, and today every major mass-manufacturer has a unit on the market.
As these same makers ramped up for digital production, digital choruses naturally joined the team. The effect as produced digitally might sound broadly similar, but it is done differently than in the analog circuits. Digital chorus pedals double a signal and add delay and pitch modulation to one path, the latter wobbling below and above the pitch of the unadulterated signal, which produces an audible out-of-tuneness when the paths are blended back together. It’s hard to fault the power and range of control that digital technology affords, and this version of the effect has been hugely successful, but many guitarists still prefer the subtle, watery shimmer of the analog version. Conversely, the same ears often find the digital variety makes them a little queasy.