Copyright laws project the "intellectual property" of creators. Fair use is the concept of limited use of a copyrighted work for commentary, criticism, or parody. Often the types of uses students and instructors use copyrighted materials for educational purposes is fair use, but there are limits.
As defined by the Library of Congress:
Copyright refers to the author's (creators of all sorts such as writers, photographers, artists, film producers, composers, and programmers) exclusive right to reproduce, prepare derivative works, distribute copies, and publicly perform and display their works. These rights may be transferred or assigned in whole or in part in writing by the author.
Public Domain is not a place. A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.
Fair use is an exception to the exclusive protection of copyright under American law. It permits certain limited uses without permission from the author or owner. Depending on the circumstances, copying may be considered "fair" for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship or research.