Copy-Paste Plagiarism occurs when a researcher takes a source and copies it word for word without the use of quotations or citations. This could be only part of a source, or a source in its entirety.
Word switch plagiarism occurs when a researcher replaces words within a copied passage with synonyms and without changing the structure of the passage. This is considered plagiarism even if you include a citation.
Accidental plagiarism usually occurs when a researcher has not kept their sources and research organized. This might result in misquoting a source, or forgetting to cite a source.
Self plagiarism occurs when one uses a work they had previously created (either in part or entirety) for another assignment.