Knowledge is created in small, incremental steps. To create new knowledge, scholars and researchers begin by reading the work of others to help build a solid foundation of facts and ideas for their own work. These building blocks are carefully documented so that researchers and scholars know who had an idea first, how it came to be accepted, and how it was it built upon by others. For this reason, there are detailed citation systems in place for giving credit where credit is due.
The purpose of citations is to provide readers with all the information they need to find the source of an idea or piece of data (i.e., who wrote it, and when and where it was published). Different disciplines—the natural sciences, the social sciences, and the humanities—have preferred citation styles that organize this information differently. Your instructors will usually tell you which style manual to use. If they have no preference, you should still choose a style and use it correctly and consistently.