Guide to Safe Driving in Korea
If you're planning on driving while you're in Korea - which many people will strongly advise against - you'll need either an International Driving Permit (if someone tries to sell you an "international driving license," don't buy it) or a Korean Driver's License. To get an IDP all you need is a driver's license from your home country that will be valid during the entire year that you want an IDP. You can't get it in Korea, though - you'll have to get it shipped from home.
If you want a KDL, you'll have to pass a 20-question driving test, in less-than-masterful English. It's not easy, though not because the question content is necessarily difficult. Friends have told me that they would have known the answers to all of the questions if they could actually figure out what the question was asking. (If you fail, you'll have to pay to take it again.) To prepare for it, you can buy the "Manual for Safe Driving" by the Korean Road Traffic Safety Authority (for something like 15k won); or you can just use the "Guide to Safe Driving in Korea", based on the KRTSA's manual but prepared by the US Military.