A citation is a reference to a source of information. Each one should contain enough information to allow a research to locate the item referenced.
The ability to interpret a citation is a key research skill. The first step it to determine the type of item being referenced. For our purpose these include:
Many disciplines have their own style guide. You may be familiar with some of these: Chicago Manual of Style and the MLA Style Guide. The definitive guide in Chemistry is ACS Guide to Scholarly Communication.