Copyright Versus Plagiarism
Plagiarism is claiming someone else's work as your own.
Breaking copyright law: Copying an entire book without receiving permission from the copyright holder. It is not enough to acknowledge the original source and who the author is. You must ask for permission.
Committing plagiarism: Inserting a paragraph written by someone else into your own writing without giving credit to that author. You didn't break copyright law because using a paragraph from someone else's writings falls within the boundaries of fair use.