As devnul3 says, a power-only cable is a USB cable where the data pins are not connected. The main reason they exist is because they can be made ever so slightly cheaper by virtue of only needing 2 connections instead of 4 (and also because you don't need to worry about preserving the quality of the very high frequency signals on the data lines).
This said, there are cases where a power-only cable is preferable to a "fully featured" USB cable: when you want to charge your device from some random USB connector somewhere. As is becoming more widely known, a USB connector can be used for many things besides the ones the user expects, so if you use a "normal" USB cable, you may end up unwittingly providing priviledged access to your device.
IOW when you're charging from a USB connector that you have no reason to trust, a power-only cable is not only ever-so-slightly cheaper, but it's also a bit safer.