Koreans see business less as a legally based interaction than a relationship. Consequently, there is a much weaker sense of law in Korean business relations than in international business. For many Koreans, a contract is part of the symbolism involved in beginning a relationship, and "beginning" is the important word. The contract thus is only as binding as the personal connection. It is not surprising, therefore, that foreign instructors in Korea occasionally have contract disputes with their employers. The employer may, indeed, consider the contract a simple working agreement, subject to change, depending upon the circumstances –– and usually after the foreigner has arrived in Korea. Most Koreans do not view deviations from a contract as a "breach," and few Koreans would consider taking an employer to court over a contract dispute.
Instead, Koreans tend to view contracts as infinitely flexible and subject to further negotiation. Furthermore, the written contract is not the real contract; rather, the unwritten, oral agreement with an employer is the real contract. You should bear these factors in mind when you sign a contract.
A basic contract for a teaching position should include provisions for the following: salary; housing; working hours; severance pay; income tax; medical insurance; and ticket home. If these items are not covered, you should negotiate until they are specifically included in the contract. Note that class size is not usually specified in a contract, although you may want to clarify this point. Private institutions generally have classes of from 10 to 15 students, while universities may have as many as 100 students in a class.