While there are dozens of different citation styles, all citations, regardless of style, will contain substantially the same bits of information. Differences in styles come down to formatting differences in the order of the information elements, capitalization, punctuation, and presentation of the author’s name, as well as how the references are presented within the text. Most citation styles can be broadly classified into two systems: 1) parenthetical (in-text) citations and 2) numbered citations.
Parenthetical styles that have an author-page format (such as MLA) emphasize page numbers (or line numbers, sections, or other markers) rather than publication year because the disciplines that use these styles make heavy use of quotation and emphasize the researcher’s reading of a particular text and edition. On the other hand, author-date parenthetical styles (like APA) are used in disciplines that place particular emphasis on the currency of information and/or thought.
Numbered citation styles (like Chicago Notes or AMA) are used in disciplines that cite a high number of studies for reading ease, and also in disciplines that tend to use sources whose citations don't fit neatly into parenthetical references (government documents, legal cases, primary documents, etc.)