A citation is a reference to the source of information used in your research. Any time you directly quote, paraphrase or summarise the essential elements of someone else's ideas in your work, an in-text citation should follow.
An in-text citation is a brief notation within the text of your assignment which refers the reader to a fuller reference at the end of the document that provides all necessary details about that source of information.
Generally, using the APA style requires the name of the author(s) and the year of publication (with no punctuation between the two items) plus page number(s), if quoting. Page numbers are preceded with ‘p.’ for a single page (e.g. p.5) and ‘pp.’ for a range of pages (e.g. pp. 5-10).
You should always reference a source when:
The exceptions to using references are when you are referring to your own opinions or research and when the information is common knowledge.