Plagiarism is copying and taking credit for the words of another.
For those concerned with issues of plagiarism, here are some excellent resources for historians, writers, students and teachers. You'll find out how to avoid plagiarism, how to detect plagiarism, as well as some basic definitions of plagiarism. In short: don't take credit for someone else's work or words.
Plagiarism: What It Is and How to Recognize and Avoid It:
A useful resource from Indiana University that not only defines plagiarism but gives helpful hints for how to tell the difference between acceptable and unacceptable paraphrasing, what is "common knowledge" and therefore does not have to be footnoted or otherwise cited, and other helpful clarifications.
What Is Plagiarism?":
Definitions and examples of plagiarism. "[P]lagiarism is an act of fraud..." Quite clear.
Plagiarism and the Web:
A guide primarily aimed at teachers on how to avoid plagiarism and how to discourage it among students. One of the best strategies is the simplest: let students know that you know about essays available on the Web -- and then, if you get an essay that's suspicious, check it out against those resources!
This site tries to "level the playing field for all students" by allowing educators to test papers against a database of other papers. The related fee-based site, TurnItIn.com, allows students and educators to submit papers for plagiarism assessment.