There are many different conventions, or approaches, to effective referencing, depending on the referencing style being used, and these can be separated into three standard systems for citing sources:
There are different versions of each of these approaches, some of which are discipline-specific. For example, APA is an author-date system that is specific to Psychology.
Be aware, there are many versions of the Harvard and Vancouver referencing styles, there is no single 'correct' style. If you refer to more than one source for guidance on Harvard, for example, you may notice inconsistencies so always try to stick to one source for guidance. The most important thing is to remain consistent.
This style uses an approach in which an author-date are located / identified directly within the text when a source is used, which then allows the reader to find the full reference to the source at the end of the essay on a separate references list (or bibliography).
This style uses a numbering system, where an alpha-numeric figure identifies that a source has been used/referenced. The reader can then locate the full reference in the reference list at the end of the essay.
This style uses footnotes or endnotes, where the full details of the source are given in the footnote (at the bottom of the current page), or in the endnotes (at the end of the essay). If the same reference occurs again an abbreviated form is used in the footnote/endnote. All references also appear in full on a separate references list (or bibliography) at the end of the essay.