There are different types of plagiarism: intentional, unintentional, and self-plagiarism.
Intentional plagiarism is self-explanatory: text is directly copied into a paper without attribution.
Unintentional plagiarism is the majority of plagiarism cases. This involves:
- forgetting to cite a quotation
- failure to mark a quotation as a direct quote
- improperly summarizing or paraphrasing content
- improperly citing a resource
These are not the only ways one can unintentionally plagiarize, but they are the most common.
There's also self-plagiarism, which is using a work written for one class and turning it in for credit in another class. It's self-plagiarizing even if some edits have changed. Always speak to your professor if you want to use a project from one class as a "stepping-stone" for another project!