What exactly is the difference between a 110v and a 220v labeled power outlet?

As far as I can see, the outlet is just a piece of plastic with some metal connecting to the power cables. Both types of outlets seem to be identically build.

Does it pose a problem to mount a 110v power outlet on a 220v power cable?

To clarify, the plugged in devices are using 220v as the power system provides, the question is really just about the power outlet itself.