A case citation provides information about where to find the text of a case. In the olden golden days, citations were only for print reporters, but now there are citations for online services as well (Westlaw, Lexis, and others.)
Print Reporter Citations
A print reporter citation has a volume number, reporter abbreviation, and page number.
The following Arizona case is published in two print reporters (Arizona Reports and Pacific Reporter) and therefore has two citations (parallel citations).
To find this case in Arizona Reports you would look in volume 198 on page 280.
Online Services Citations
When an opinion is issued by a court, a copy is obtained by print reporter publishers, Westlaw, Lexis, and a number of other legal publishers. Since it takes a while for hard copy citations to be assigned to opinions, Westlaw and Lexis needed some way to identify their cases so they both created their own online citations to assign to cases.
A Westlaw citation is comprised of a date and a number (example - 2020 WL 5581701). Westlaw citations are removed when cases are assigned hard copy citations. If a case is never reported in a hard copy reporter, the Westlaw citation remains on the case permanently.
A Lexis citation is comprised of a date, number and also court information (example - 2020 Ariz. LEXIS 295). The citation is permanent and is not removed when hard copy citations are put on the case.
The National Reporter System is the set of case law reporters published by West Publishing which includes all state and federal cases.
State Regional Reporters
State cases from the 50 states and the District of Columbia are published in seven regional reporters: Atlantic, North Eastern, North Western, Pacific, South Eastern, South Western, and Southern Pacific.
From Wikipedia, here is a list of the reporter citations, series, years covered, volumes, and states covered.
Individual State Reporters
Approximately 30 states have their own individual reporters. For example, Arizona Reports contains ONLY cases from Arizona Courts of Appeal and the Arizona Supreme Court.
West publishes U.S. District Court, U.S. Court of Appeals, and the U.S. Supreme Court cases in separate reporters.
In many jurisdictions, court rules limit which opinions are published and can be cited as legal precedent.
Arizona Rules of Civil Appellate Procedure
Arizona Rules of Civil Appellate Procedure Rule 28 controls publication of Arizona Court of Appeals opinions.
Rules of the Arizona Supreme Court
A.R.S. Sup.Ct.Rules, Rule 111 controls the publication of Arizona Supreme Court opinions.