Citations are used both to provide information about the source you are citing and to make sure the person reading your work can find the source. Concord Law School uses the current edition of the Bluebook (Uniform System of Citation) to determine the format of citations. For example, the citation of Kelo v. City of New London is:
Kelo v. City of New London, Conn., 545 U.S. 469 (2005).
The first part of the citation is the case name/title/caption/party names. Lawyers often refer to cases by the title. When scanning your document to see what sources you cited, they will probably recognize the more important cases by the titles. An incomplete citation not only makes your document look less professional, it undermines the impact of your arguments by making it so a reader would have to research your citations (in other words, they might simply discount your arguments rather than do a lot of research).
The second part of the citation is what is sometimes referred to as the "book cite." This is the address, if you will, of the case. The 545 in the citation above refers to the volume of the reporter where the case was published. The U.S. is the abbreviation of the reporter in which it was published. You may find the abbreviations in the Bluebook or online at http://lib.law.washington.edu/ref/repdig.html. The number, 469, refers to the first page of the case. Case citations end with a parenthetical. In this case, it is (2005). This is the year the case was decided. If it was not apparent from the reporter what court ruled, there would also need to be a court abbreviation in the parens preceeding the year.