A word-for-word copy of someone else's work, without attribution and quotation marks, is deliberate plagiarism.
Self-Plagiarism
If you have already submitted something you've written for an assignment for any other class, using that same text without instructor permission and proper citations is plagiarism.
Patchwork
Taking bits and pieces from a variety of sources, putting them together, and presenting them as your original work is plagiarism. You need to cite your sources.
Accidental
Sometimes you forget. Sometimes you don't realize you paraphrased. Accidents happen, but it's still plagiarism. Keep careful track of your sources!
Are you plagiarizing?
A flowchart depicting a general guide to understanding written plagiarism.
Question: Are your own words being used?
Yes
Q: Is it your idea?
Yes Yay! You’re not plagiarizing!
No. You’re paraphrasing, and need to add a citation and bibliography.
No
Q: Are you using quotation marks or placing it in blockquote?
Yes? Yay! You’re not plagiarizing!
No? You’re plagiarizing! Go quote it! Once you are done be sure to add a citation and bibliography!