Plagiarism is the taking and using as one’s own the thoughts, writings, or inventions of another (definition based on the Oxford Dictionary of the English Language, s. v. “plagarize”). Rutgers University considers plagiarism a very serious offense, a violation of academic integrity — see Policy on Academic Integrity for Undergraduate and Graduate Students.
For help in determining how to give credit where credit is due when you use the works of others, see the unofficial handbook of the History Department: A Student’s Guide to History, 7th edition, by Jules R. Benjamin (Boston: Bedford Books, 1998), “Avoiding Plagiarism” and “The Art of Paraphrasing”, pp. 83-84. Both The History Handbook by Carol Berkin and Betty S. Anderson (Boston: Houghton Mifflin Co., 2003), Chapter 10, “What Is Plagiarism and When Would I Be Cheating on My Paper?” (pp. 97-102) and A Pocket Guide to Writing in History, 5th edition, by Mary Lynn Rampolla (Boston: Bedford/St. Martin’s, 2007), Chapter 6, “Plagarism: What It Is and How to Avoid It” (pp. 88-95) offer good advice.
Of interest also is the interactive tutorial, “How to Avoid Plagiarism,” a product of the Paul Robeson Library Reference Department and funded by a Rutgers Camden Provost Grant for Undergraduate Education.