Western Japan is 60Hz (same as the US), Eastern Japan is 50Hz.
Voltage is 100v all throughout, the only US thing I couldn't get to work was a popcorn popper.
The plug is basically the same, though they don't have the 3prong outlets and the prongs are the same size (some US plugs are the same, others have one prong that is slightly bigger). For the most part outlets are made to accept both.