You plagiarize when you take another author's ideas or words, and use them in such a way that it appears that the ideas or words are your own.
You take someone else's words. If you use another person's language, then you must notate explicitly (using quotation marks or block quotes) so that you clearly indicate that the words are someone else's, and not yours.
You use someone else's idea or finding. The critical point here is that any time you cite a fact that is not common knowledge, use an insight or thesis, or follow the structure or plan of another person's writing, you must cite the source for that idea, even if you use your own words to describe it.
It is not always easy to decide which facts are considered common knowledge. When in doubt, cite a source. It does not matter whether you intend to plagiarize; if you use someone else's words or ideas without making it perfectly clear that you're doing so, you have committed a serious breach of the Mount Holyoke honor code.
There are also offenses that are not plagiarism but that can be very serious. You must make sure that any source is an appropriate one and that your citation conforms to proper scholarly procedures. For example, even re-using your own writing from one class to another is a violation of the Honor Code unless you have permission from your instructor. If you don’t know whether a source is appropriate or what constitutes scholarly procedure, consult your professor.
Keep in mind that collaborating with other students while writing a paper is not in itself plagiarism, and may be wholly appropriate. Ask your professor for guidance about what kinds of collaboration are allowed for a particular course.
What to cite and how can vary by discipline, but some rules always apply. Visit this page for more on discipline specific rules of citation.