Citations are a necessity for any piece of legal writing, and not in abstract terms. Citation is the key purpose of legal research, as the presence of citation indicates not only research but also authority, which can emphasize the strength of the material used to support the argument.
Typically, what a piece of legal writing will always do is cite a primary source (like a law or legal statute), since that will represent what the argument will be based upon or around. Opinions, precedents, and the like are derived from case citations, in that they are always taken from a case name, which is described by the parties involved in the case (For example: The United States of America v. John Doe).
As a general rule, legal citation boasts the most complicated means of citation in almost all of writing. For instance, if we refer to our hypothetical case of the United States of America v. John Doe, which was published in volume 814 of US Reports, happened in 1977, and would like to reference pages 47-48, then it would like this:
U.S.A. v. John Doe, 814 U.S.
If we were specifically citing the minority opinion in the case, which may have been written by Justice Jones (with all the other information remaining constant):
U.S.A. v. John Doe, 47-48 (1977)
Means of citation can become much more complicated, especially in dealing with cases in various jurisdictions as well as differing forms of publications (where these citations have to account for number of edition, and so forth). In the advent of legal databases such as Lexis-Nexis, these citations have grown in complexity, but the electronic means of generating them have become more common.
Ensuring that a legal document has the proper citation is one of the most important elements in drafting a legal argument, just like a criminal case has to check all evidence, a legal argument must be verifiable by the members of the legal institution that read the document as part of a legal proceeding.
The two books most recommended for those looking to learn about legal citation is The Association of Legal Writing Directors (ALWD) Citation Manual: A Professional System of Citation and The Bluebook: A Uniform System of Citation.