In order to plug items you bring from North America into most Korean outlets, you have to use a an adapter, a converter, or a transformer.

DO NOT plug in anything with a simple cheap adapter. At best, it will ruin whatever you've plugged in, and at worst, it will catch fire. Coverters (110/220). Converters (110/220) will generally tolerate the stress of a dim lamp or a small radio, but DO NOT plug your computer monitor or hair dryer into one. A Transformer (110/220) is larger and heavier, and it costs anywhere from $40-$100.

Adapters and converters are much cheaper, but almost entirely useless. You can buy adapters and converters at most any hardware store in North America. However, Transformers are a bit harder to find. I bought mine on E-bay for about a third the price of big-chain hardware stores before I lived abroad most recently. They are also available in Korea.

Note that the terms 'converter' and 'transformer' are sometimes used interchangeably, so here are some photos of what I'm talking about:

X An adapter    X A converter

      Transformers

Note that if you have a relatively new computer, the converter / transformer may already be part of the power cord. Check with the manufacturer as to whether this will be sufficient on its own to plug it into a 220V plug with a cheap adapter.