Korea apparently is much worse and will likely literally FRY any equipment made to Japanese or USA standards without a converter.
Here are two excellently written sites I found in my quest to protect my precious Ipod Nano.
☆ Detailed info on Japan's situation including pictures and explanation of area variations.
☆ Worldwide Voltage and Frequency table including linked pictures.
In summary, Japan is complicated - I'm not summarizing it - but USA is a fixed at 120 V with 60 Hz frequency while Korea is mostly 220 V with 60 Hz frequency.
Thanks again for stopping into my little corner of the 'net, and Happy Browsing!!