According to visa waiver agreements and visa-free entry program, most foreigners (in 155 countries) can enter South Korea WITHOUT a visa as long as the purpose of their visit is tourism or temporary visit. You can find full details about the visa-free entry of foreigners on this page. It may be sufficient to read the first two sections, titled “Countries under visa waiver agreements” and “Designated visa-free entry”.
Passport holders of some countries (including India and China) are required to get a visa before coming to South Korea. If you do need a visa invitation letter, please send an email to Kyungsik Han (Local Arrangement Co-Chair) and cc Junehwa Song (General Co-Chair) with the following information. Also, please refer to this page for further information about visa applications.