It is expected that you consult and use others' research in your writing; however, when you do, you must cite the source of the information. To fail to do so is plagiarism. The following paragraphs should help you determine when you must cite and when a citation isn't necessary.
When to Cite
Whenever you are presenting the words, ideas, images, or data of someone other than yourself, you must cite the source. This includes paraphrases, because even though the words are your own, the idea you're presenting is not. When paraphrasing, ensure that the wording is actually your own; simply rearranging a few of the author's words or replacing them with a few synonyms is not paraphrasing and constitutes plagiarism. When quoting directly, place quotation marks around the author's exact words.
When Not to Cite
If the wording is your own and the idea being expressed is your own, no citation is necessary. It is not plagiarism, as you are not presenting the ideas or words of someone else.
In addition, if an idea or fact is widely known and not disputed, it is considered common knowledge and does not need to be cited. This information is generally known by everyone within the discipline and can be found in numerous sources. When unfamiliar with a discipline, as many students are, it can be difficult to know what is common knowledge and what is specialized knowledge that requires a citation. It is always best to err on the side of caution. When in doubt, cite it.