A citation is the way you tell your readers that parts of your work came from a source other than you and will also provide them the information necessary to find that source on their own to read and verify the information.
ANY time you use information that is not your own.
Remember, "When in doubt, cite it out!"
There are plenty of citation resources out there to help with all your citation needs, including how to cite in different writing formats such as APA, MLA, or Chicago to name a few. There are even a few websites that can help you with your bibliography (which is a page that lists out all your cited works at the end of your paper).