When you refer to another person's work in your writing, you are required to cite its source. Not doing so is plagiarism.
In the text of a work, use a parenthetical citation, footnote, or endnote. At the end of the document, use a list of sources in a bibliography, works cited, or reference list.
English classes use the MLA style (Modern Language Association); most other classes use APA style (American Psychological Association). Check with your instructor as to which style to use.
If you need assistance with citing resources, use Ask a Librarian.