Plagiarism is using the ideas and words of others without clearly and properly acknowledging that those ideas and words came from someone else. The term "plagiarism" is derived from Greek and Latin words for "kidnapping."
"Plagiarism is presenting as one's own in whole or in part the argument, language, creations, conclusions, or scientific data of another without explicit acknowledgement."
- WWU Academic Honesty Policy and Procedure
Plagiarism includes directly copying someone else's words as well as paraphrasing their ideas without proper attribution.
Keep in mind that students can commit self-plagiarism, also known as "double dipping." Self-plagiarism includes submitting the same paper in response to two different assignments. Once you've submitted a piece for a specific class it can no longer be considered original work in the context of another class. If you would like to reuse your work, make sure to talk to your professors and/or consider citing yourself.
Also, be aware that many professors use plagiarism-detection programs like "TurnItIn" that check your papers against other student papers, websites, and online articles.