A word-for-word copy of someone else's work, without attribution and quotation marks, is deliberate 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.
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.
Sometimes you forget. Sometimes you don't realize you paraphrased. Accidents happen, but it's still plagiarism. Keep careful track of your sources!
Avoiding Plagiarism from PALNI on Vimeo.