There are two main categories of citation styles, those that are known as In-line (aka In-text) citations and those that use either Footnotes or Endnotes
First, an example of In-line cites:
From the paper prose:
Studies have been conducted on various types of Kryptonite, including Red Kryptonite (Castillo, 1976), Green Kryptonite (Adams, 1995) and Blue Kryptonite (Barber, 1987).
This might result in a list of References as such:
Adams, RA (1995) Study of green kryptonite, Journal of Kryptonite Studies, 14:1, 567-578
Barber, NH (1987) Study of blue kryptonite, Extraterrestrial Crystals, 16:7, 234-257
Castillo, XA (1976) Study of red kryptonite, Journal of Alien Mineralogy, 7:8, 1-15
So, the prose of the paper would be peppered with surnames and publication years, while the bibliography or list of references would be organized alphabetically by the author, in these cases surnames, but also sometimes agencies and organizations.
So, lets 'translate' the above into an Endnote type style:
From the paper prose:
Studies have been conducted on various types of Kryptonite, including Red1, Green2 and Blue3.
This might result in a bibliography as such:
1. Castillo, XA (1976) Study of red kryptonite, Journal of Alien Mineralogy, 7:8, 1-15
2. Barber, NH (1987) Study of blue kryptonite, Extraterrestrial Crystals, 16:7, 234-257
3. Adams, RA (1995) Study of green kryptonite, Journal of Kryptonite Studies, 14:1, 567-578
Notice that the reference numbers in the bibliography correspond to the superscript numbers in the prose, and that the sources are presented in order of their appearance, rather than in alphabetical order by author, effectively reversing the listing from above