What's the proper voltage for a USB charger?

I have several chargers (for normal batteries, as well as PDAs etc) that have a "USB" style plug. Some of which state a 5V output, and some say 12V output. Since I do not wish to fry my newly purchased iTouch I found a 5V one that does works (A Kensington multi-prong plug with a USB charger plug-in). I just wonder why there would be 2 voltages for a single USB standard? Would I actually harm my ipod if I tried to charge it with a 12V charger? Anybody has any luck/horror stories with those tiny cube-like chargers that you can buy on ebay that look like they were made for ipods? Just curious.