A citation is an acknowledgement that you have used someone else's research or ideas in the creation of your own research paper or essay.
In addition to giving credit to authors for using their work, citations allow readers of your work to track down sources that might be of interest to them.
From the Purdue Writing Lab
Whether you are quoting directly or paraphrasing, you make a brief in-text citation at the point in your writing where you use another person's work. In APA style, this generally includes mention of the author, the date of the publication, and page number, when necessary.
The References page goes at the end of your research paper and includes full citations for all of the sources you consulted.