To avoid plagiarism, the History Departments encourages students to
a) use another person's work word for word only when putting the passage in quotation marks;
b) avoid copying key words or phrases from the original when gathering notes; otherwise, one is likely to transfer the same into the paper;
c) identify the author and source of words, ideas, etc. when paraphrasing;
d) acknowledge to the teacher the help received from others who have contributed to one's understanding of or completion of an assignment.
Source: History Department Policies
According to the Burroughs's Academic Integrity Statement, plagiarism results when a student passes off as one's own the ideas, words, thoughts, etc., of another; it is the use without credit of the ideas, experiences, productions, or work of someone else; it is an act of intellectual dishonesty.
This means you have borrowed another's words or ideas, and have not acknowledged that you have done so. You can avoid plagiarism by citing your sources!
Here are just a few reasons why cheating (which includes plagiarism!) is wrong:
Source: Academic Practices, School Culture and Cheating Behavior, by Gary Niels