- 선생님 - teacher. Has a sense of respect (a teacher is generally respected).
- 교사 - a dry word referring to teacher as a profession (not used much in daily life).
- 선생 (without 님): similar to 교사 but more common. Sometimes used derogatorily.
(the above words usually refer to teachers at a grade or high school)
- 교수, 교수님: professor (at a college or university)