In addition... you can get 500mA ones and 1000mA ones, unless the instructions for the device say it's OK to charge at 1000mA it's safer to get a 500mA one to avoid the risk of damaging the battery.
You plug any devices' USB cable into its socket and the other end into your Mp3 or other device. The plug puts out the same voltage as your PC's USB socket. The advantage is that it does the job regardless of the type of socket on the device to be charged