Copyright, broadly understood, is the legal right a person has to keep his or her work from being copied by others. Say, for example, a musician records a song and distributes it. Either the musician or the distribution company (recording company, etc.) will own the copyright to that song. You aren’t able to take that song, say that it’s yours, and profit off the sale of it. To do so would be a breach of copyright and would make you legally (and perhaps monetarily) liable.
Copyright is a subsection of the larger discussion surrounding plagiarism, and the two often intersect. For example, by not respecting copyright, especially the copyright of an author of a scholarly work, you are effectively committing plagiarism. Their work should be properly attributed through citation.
Yes! There are occasions where it’s okay to use part of a copyrighted work. These occasions are gathered under the title “Fair Use” (a more detailed discussion of Fair Use can be found in CUNY’s guide on the subject and on the website of the U.S. Copyright Office).
Generally speaking, Fair Use is applicable
In cases where you want to excerpt a portion or portions of a work for the purposes of criticism or discussion
In cases where the intent is to parody an already existing work
In an academic context, Fair Use cases will most likely involve quotations from scholarly sources. As mentioned above, properly citing a source avoids any violation of an author’s copyright.
The following answer is excerpted from CUNY's Fair Use and Copyright LibGuide:
Yes. The student is copying the entire work – not a portion or excerpt. One chapter is generally considered Fair Use. While textbooks are undeniably expensive, this unauthorized duplication undermines and hurts potential profits of textbook publishers.