In addition to the Fair Use exemption, of importance to those who teach in a professional capacity is the Face-to-Face Teaching exemption (section 110 of Title 17). In summary this exemption allows for legally reproduced work (e.g., articles, small portions of books, short video clips) to be used in a classroom or similar place devoted to instruction in the course of face-to-face teaching activities of a nonprofit educational institution. The exemption applies as long as there is no direct or indirect admission charge, it is a regular part of the instructional activities and it is directly related to the teaching content.
This means you cannot show a video that has nothing to do with your course or copy a cartoon onto a transparency or PowerPoint slide show or even put it in the syllabus just to entertain your students. It must relate to the course content.
For more information concerning § 110. Limitations on exclusive rights: Exemption of certain performances and displays, visit these Web pages