As soon as you use the ideas, words or other intellectual property from an outside source, acknowledge the original work by citing it. In-text citations distinguish other people's words and thoughts from your own and direct people to the complete citation at the end of your paper. The same format works for quotations, summaries, paraphrases, etc., but formats vary depending on whether or not you mention the author in the sentence. Format will also vary from style to style.
Here are examples using MLA style.
Author included in the sentence:
Format: include page number(s) in parentheses at the end of the sentence
Example: Freud claims that blah blah blah (152-157).
Author not included in the sentence:
Format: include author's last name and page number(s) in parentheses at the end of the sentence
Example: Blah blah blah, blah blah (Freud 152-157).
For examples of endnotes, footnotes, and more parenthetical citations using many more styles, see Research and Documentation Online.
Want printable citation guides with more examples?
If I only knew then what I know now...
"I realize after having taken this class that I wasn't even citing things right, that I was doing a lot of things wrong!" A response from a graduating senior when asked if there was anything that she wish she had known before taking English Composition 1-2-3 (Information Literacy and Research Skills). Accurately documenting her sources was just one of a long list of things that she regretted not learning about until her last quarter.
(Anonymous Student Interview 30 May 2003)