from Merriam-Webster's Dictionary
Plagiarism is a serious intellectual offense. However, if you are aware of what constitutes plagiarism, and strive to avoid it, there is nothing to be worried about.
Here are four common types of plagiarism:
Direct plagiarism occurs when a student copies the work of another author verbatim without citing the source and without using quotation marks. Think "Cut-and-Paste".
from Bowdoin College: An Example of Direct Plagiarism
Self-plagiarism occurs when a student "Recycles" material from another one of his or her own papers without citing that paper as a source.
Mosaic plagiarism occurs when a student "Cherry-picks" words, phrases or more general concepts from another source, then finds synonyms or changes the wording slightly, without acknowledging the source.
from Bowdoin College: An Example of Mosaic Plagiarism
Accidental plagiarism occurs when a student forgets or neglects to properly cite his sources. Not knowing what consititutes plagiarism, making errors in attribution, or simply forgetting still results in an offense.