A citation might be written in several formats. The two most common formats are the APA and the MLA style.
If the subject being researched is a social science, citations will tend to be written in the APA style. APA stands for the American Psychological Association.
If the subject being researched is literature-focused, the citations will tend to be written in the MLA style. MLA stands for the Modern Language Association.
This tutorial uses both formats.