Citations are how you credit a source of information. They tell your audience where you got your information, so they have the option to check it for themselves.
Citations are also how you avoid plagiarism by not passing the work off as your own. You can read more about what constitutes plagiarism here.
This guide provides brief overviews on how to create citations using the ninth edition of MLA format and the seventh edition of APA format. It also provides a guide to using MyBib, a free service that properly formats your citations.
I also recommend using the Purdue OWL (Online Writing Lab) if you need help - especially going beyond the basics of what's covered here!
