Barcodes For your CD Release
According to the Uniform Code Council's Web site, a bar code is "a precise arrangement of parallel lines (bars) and spaces that vary in width to represent data." Specifically, they represent a unique 12-digit number, sometimes called a Universal Product Code number. When placed on a product, these bars can be scanned by lasers so that stores can digitally record what products they have sold. You've probably seen how much time they save cashiers at supermarkets.
You need a barcode if  you plan on selling your CD in a store that requires you to have one. This would include all retail chains and online stores. If you're just selling CDs at your gigs and in little mom and pop stores, then you don't need one.
 
Barcode Stickers   Good Or Bad?
Stickers are a great way to get the value of having a bar code, without letting the bar code interfere with your graphic design. They are attached to the polywrap, so the store can scan the CD, but once it's purchased, the bar code gets tossed away with the wrapping. So that may not be a good thing. All major label releases have the barcode in with the graphics on the back tray card.