If you are planning on staying in South Korea for less than 90 days, you do not need a tourist Visa. However, if your trip is longer than 90 days, you will need a Visa. Below is a useful website that you might want to look at that explains traveling information to South Korea.

http://south-korea.visahq.com/

If you're just going to visit for fewer than 90 days, and you don't plan to work while you're there, then you don't need a visa. If you want to stay longer or work, check out the Korean Embassy's helpful site on getting a visa: http://bit.ly/cxpGAU