Publication Manual of the American Psychological Association by Clemson defines plagiarism as:
A quote is the most common piece of information that needs a citation. You quote something when you re-use, either in part or in whole, a piece of information exactly as you first encountered it. Quotes are set aside in text with special markers, such as quotation marks ("") or block-style indentation. The words in the quote exactly match those from the original document, so the quotation marks are used around the quoted section, in addition to the citation.
A statement is paraphrased when new, unique words are used to describe an existing concept, argument or opinion. In this case, while the actual words written down will be your own, you need to cite the source of the information. Different from direct quotes, there are no stylistic markers for paraphrasing. This is why it is incredibly important to make sure citations are included, informing your audience of the source of your knowledge. Including the citation gives credit to the original author of the idea, even though the concept is being described in a new way.
A summary is a condensed description of, usually, an entire work or idea. Summaries are written using your own, unique words, but the content is pulled from another source. In these cases you need to cite the source of the original information. The citation shows that the content of the summary is not original thought but instead lifted from another source.
Common knowledge does not need to be cited. Of course, the challenge is defining what counts as common knowledge. In professional circles, common knowledge is dictated by the audience. An audience of English Literature scholars does not need a citation when "the hero's journey" is referenced, because one can assume the audience has an understanding of that idea and knows how to look up more information if they need it. However, if you were to speak to the same audience about the insulating properties of graphene, you'll want to give a citation so that they know how to get back to the source of information, should they need to.
As a graduate student at Clemson, a shorthand to determine if something is common knowledge is this: is the information something you can reasonably assume every other graduate student at Clemson already knows? If yes, then it is common knowledge. However, if you think it is something students outside your discipline may be unfamiliar with, or it is an obscure fact, it is not likely to be common knowledge and you should include a citation.
--adapted from the Clemson Libraries Plagiarism Guide
Plagiarism can happen accidentally or intentionally. By being prepared and working ahead, you can avoid some of the common pitfalls that lead to plagiarism.