Copyright is a vague and ambiguous territory in which many people are nervous to tread. The more informed you are about copyright, the better you'll be able make choices about how and what copyrighted works to include in your course content.
Remember that copyright laws were created for the express purpose, "to promote the Progress of Science and useful Arts, by securing for a limited Time to Authors and Inventors the exclusive Right to their respective Writings and Discoveries" (Constitution). Copyright does not mean that intellectual property owners are granted perpetual property rights. Intellectual property is not considered merchandise. However, creators are acknowledged to be due some recompense for their creativity and effort. Thus, copyright laws give creators the temporary, exclusive right to reproduce (copy) their work and distribute those copies, publicly perform or display the work (e.g. paintings or music), or create something else based on that work (e.g. fan fiction). If someone else wants to copy, perform, or base their own work on another's, that person must get permission. And the copyright owner can charge for that permission.
Note that the rights are temporary. Copyright terms are very long at the moment (life of the author + 70 years) but eventually, works will go into the public domain where they will be free to use. In the meantime, fair use helps balance things out by giving those who intend to use the work to "promote the progress of science and useful arts", a way of doing so without asking permission. And Creative Commons allows people to give permission for people to use their work.