When doing research it is often necessary to consult other research. When you use an idea or statistic that comes from another source, you need to reference that source; failure to do so is considered plagiarism.
The citation is the reference to that original source (i.e., book, paper, article, web site, etc.).
A citation usually includes four important pieces of information:
Citing sources gives credit to the original author, enables readers to find the original source and strengthens your work.