Legal citations enable a reader to easily locate the cases, statutes, regulations or other material cited in legal documents. They also make it clear what level of court a case was at, the year, etc.
United States federal law is codified in the United States Code. The Code is arranged as 54 titles of various subject matters of federal jurisdiction (e.g. Title 12: Banks and banking; Title 17: Copyrights; Title 47: Telecommunications).
The titles are then split up into chapters, which are further divided into sections (e.g. Title 17: Copyrights -- Chapter 1: Subject matter and scope of copyright -- Section 106: Exclusive rights in copyrighted works).
State law citations tend to be unique to the state. For Massachusetts, it's Mass. Gen. Laws ch. CHAPTER#, § SECTION. A section of the Massachusetts General Laws relating to the issuance of search warrants can be cited as Mass. Gen. Laws ch. 276, § 1. However, some courts require a different type of citation in documents submitted to them. Within Massachusetts courts you may find a shorter citation for the same law, such as G. L. c. 276, § 1.
Cases often appear as a number, an abbreviation of the publication in which the document is found followed by another number, e.g. 410 U.S. 113 (1973) is the citation to Roe v. Wade. This means it is a Supreme Court case decided in 1973 and can be found in volume 410 at page 113 in the United States Reports. The full citation then, is Roe v. Wade, 410 U.S. 113 (1973).
A case could be reported in different reporters, so it would have parallel citations. These other citations can be added as an alternate way for the user to find the text of the case, e.g. Roe v. Wade, 410 U.S. 113, 93 S.Ct. 705 (1973). [S.Ct. means the Supreme Court Reporter.]
State court case citations follow the same pattern, e.g. Goodridge v. Department of Public Health, 798 N.E.2d 941 (Mass. 2003). This is a 2003 decision of the Massachusetts Supreme Judicial Court, and is found in volume 798 of the North Eastern Reporter (second series) on page 941. A parallel citation is 440 Mass. 309, meaning it's also found in the Massachusetts Reports.
For courts lower than the U.S. Supreme Court, one would also include the court abbreviation. A list of federal court abbreviations can be found here, and state court abbreviations can be found here. They can also be found in the Bluebook. Abbreviations for case reporters can be looked up at the Cardiff Index to Legal Abbreviations.
Federal regulations are contained the Code of Federal Regulations and categorized by subject matter. They are cited as TITLE C.F.R. SECTION -- for example 21 C.F.R. § 500.51 refers to Title 21 (Food and Drugs) Code of Federal Regulations Section 500.51 (Labeling of animal drugs; misbranding).
A regulation of Massachusetts relating to the use of local anaesthesia in dentistry might be cited as 234 CMR 6.15.