When writing citations, the most basic question you need to ask yourself is, “What information would someone need to find this source on their own?” Although nowadays you'd likely use your phone to take a picture of a product you want to be able to find again (your favorite brand of bread, for example), citations require a little more effort. The basic principle, however, is the same. When taking a picture to remember something, you make sure that you have the name and any other identifying information that will help you find it again. Similarly, when writing a citation, you want to make sure you include the basic identifying features of a source, namely the author, title, date, and publisher.
Citations serve many purposes. Beyond allowing others to find the sources you use, citations also give your audience clues about the type of information you've used, where that information came from, and how reliable the information is within a particular context, all without having to go find the source directly. There are two main categories of each citation style: in-text citations and full citations. In-text citations provide a reader with immediate information about the source from which a piece of information came. Full citations provide a reader with more complete information about the source.