A lot of USB chargers put out more than the 5v standard. The device they are associated with can cope with this and, indeed, may benefit from it (faster recharge). The device itself will charge from a standard USB socket (on a PC, Wii, etc) but not as fast.
The reverse may not be true. While it is safe to plug any USB chargeable device into a standard 5v USB socket, it is not safe to assume that the charger for a specific device is safe to use with another device.
Always check that the charger's output is 5v regulated DC before using it to charge something other than the device it was intended to charge.
Bomb #20: "Let there be light"