Plagiarism (from the Latin word plagiarus : "kidnapping") is an act of academic dishonesty: the use of the words*, ideas, or information of another person without giving them proper credit and acknowledgment. It is an ethical violation.
Notice the root word means "kidnapping". Plagiarism isn't just stealing a thing, it's the taking of someone's intellectual "child" -- something they created.
Plagiarism is not always intentional, but it is an academic violation regardless of intent. We like to think of Plagiarism at three levels, all of which are plagiarism and all of which have consequences.
Intentional, Pre-Meditated, Malicious: you actively choose to pass off the work of another person as your own in order to improve a grade, avoid doing work, or mislead your audience, etc. This is cheating.
Examples:. Buying a paper online or taking a paper from another student in a previous year. Getting answers to a test from a friend. Re-using work from an assignment in a different class but passing it off as new or original work for another assignment.
Intentional, Heat-of-the-Moment, Negligent: you reactively choose to pass off the work of another person as your own due to lack of foresight, improper planning, or sloppy research practices. This is also cheating.
Examples: Copying someone's homework because you forgot to do it. Not adding a citation for a piece of information because you can't remember where it came from and don't want to re-find it.
Unintentional, Unnoticed, Negligent: plagiarism that you did not intend, and may not be aware of, but is still evident in your work, and is still misrepresenting what ideas are actually yours. This is academically dishonest.
Examples: Not citing a source because you didn't know, or think, you needed to. Not properly citing a source. Misusing quotations or paraphrases in such a way that it is unclear what is your idea, and what came from another source.
You use ideas, words, or information from a source and do not make note of it in your text and in your works cited. This includes both direct quotation AND paraphrases (putting ideas into your own words).
You use the exact words of another person or source without making clear which words are not yours.
You attempt, poorly, to put someone else's ideas or words into your own words. Using different words ("thesaurus-izing") is not the same as using your own words, even with citation.
You attempt to cite a source, but it is unclear what material is from the source and what is your own writings. You cite a source, but it does not correspond to an entry in your works cited. You cite a source that is made-up.
You rely too heavily on a particular source, or too much of the text of your work is from other texts.
"Double Dipping": directly re-using your own prior work/papers/etc with out citing it. (Yes, you CAN plagiarize yourself). Example: Turning in the same work for different assignments, or for different teachers (with out first obtaining permission from both teachers).