"Your job (working at a Korean University) will often consist of teaching mandatory Freshman (and sometimes Sophomore) English courses. Schools vary in how they administer this, but it can involve one 3-hour per week course during either the student's first or second semester, or it could involve something like four 1-hour per week courses over the first two years of a student's program. You will sometimes be able to teach other classes like writing, presentations, or business English to students in majors like business, airline business, tourism or English. It really depends on the university. Student English levels range from almost fluent to very low. All of these classes involve designing a syllabus, administering tests and homework, and giving final grades."
I would add that what classes you teach can really make a big difference in terms of job satisfaction. While you often don't have much say in your first year or two and will have to take whatever you get given to you, if you can move up in the world into teaching things besides mandatory freshman English, your job will probably be much more rewarding, although more will be required of you in terms of preparation and grading.
For example, at my current university most of the teachers teach 9-12 (separate!) classes of around 30 students only once/week for 50 minutes. It's almost impossible to get to know the students and it's not easy to develop a friendly relationship. For a "real" teacher who is serious about their teaching, it can be quite demoralizing. But, it is a very easy job and if you are pursuing other interests outside work, it can be quite ideal since you'll have to expend very little in the way of mental energy.