A "citation" is the way you tell your readers that certain material in your work came from another source. It also gives your readers the information necessary to find that source again. This information includes:
http://research.lhup.edu/citation
Giving credit to the original author by citing sources is the only way to use other people's work without plagiarizing. But there are a number of other reasons to cite sources:
http://research.lhup.edu/citation