When you use someone else's words or ideas, you need to give credit to the author by citing the original source. Plagiarism is falsely claiming ideas, research, theories, or words are your own. Plagiarism is a serious academic offense. The most common kind of plagiarism results from incorrectly citing a source or not realizing that the source needs to be cited.
There are a couple different styles to cite information, such as using footnotes or in-text citation. Your teacher will tell you which style and format to use. The three most commonly used styles are APA, MLA, and Chicago.