A citation, or reference, is the information that identifies a source.
Consider this: If someone tells you about a great book and you want to read it, what information will you need to find it? Knowing that the cover is blue won’t be enough. At the very least, you’ll need the title. If it’s a common title, you’ll need the name of the author as well. If your friend is recommending a particular edition of, for example, Janson’s History of Art, you may also need the publisher, the publication year, and the edition number. That’s a citation.
The information that identifies a book or any other source is called bibliographic information. The pieces of bibliographic information needed to identify and find a source vary depending on the type of source. Here are some common types of sources and the information needed to identify them:
Websites change constantly, and URLs change too. If a URL no longer works, your audience can search for the page using the author, page title, and site title.
Sometimes the information truly isn’t there: some articles don’t have authors listed, some product catalogs don’t include a date, and some websites don’t provide much information at all.
First, make sure that you have searched thoroughly for the missing information, and use your common sense. If you can’t find publication information in the front of a book, look in the back. If a web page doesn’t list an author, look for an “About” page. If a website doesn’t list a responsible party or a date when the information was last updated, consider whether you want to rely on a nameless, un-dated source for your information.
If you decide that the source is reliable but you can’t find a piece of bibliographic information, simply leave that element out.