Referencing is a system that allows you to acknowledge the contributions and work of others in your writing by citing your sources. A feature of academic writing is that it contains references to the words, information and ideas of others.
All academic essays MUST contain references. Referencing guards against plagiarism, a serious academic offence. Plagiarism is copying someone else's words or ideas and presenting them as your own.
See Plagiarism and academic integrity
Make sure you are familiar with the referencing style your faculty or school requires. Most have guides specifying the system they prefer. Often Schools/Faculties don't mind which system you use as long as it is consistent. If this is the case, use the system you are most comfortable with.
See The Learning Centre guides to various citation styles
Remember to list all the books and articles you use for the essay in a Reference List. This is a list of all works cited in your essay and should be the final page.