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!
How Plagiarism Happens, and How to Avoid It
Deliberate
When you attempt to pass off the work of someone else as something you wrote or created, you are plagiarizing. Some examples of deliberate plagiarism are:
intentionally copy word-for-word from books, articles, or online sources without citing your source
buying, downloading, borrowing, or copying another person's paper
not providing attribution to images or charts that are not designated as free to use without attribution
Accidental
Sometimes students make a mistake when including sources in their papers. Here are some examples of things that can lead to accidental plagiarism:
Using too much of the original wording when paraphrasing
Building on an idea without crediting the original idea
Another person edits your work and unknowingly makes changes that are too close to the source
Improperly citing the source, or citing the wrong source
Solutions
Some strategies you can use to avoid plagiarism include:
create a list of full citations as you use the source, rather than waiting until the end to create the citation
start your assignment early, so that you do not make mistakes when you are pressed for time