The Bluebook is the style manual that governs how American legal documents are cited in legal memoranda, court documents, and law journals. It is published by the editors of the law journals at Columbia University, Harvard University, the University of Pennsylvania, and Yale University.
As of this writing, The Bluebook is in the first printing of the 20th Edition.
The Bluebook is not the only legal citation manual available. The Association of Legal Writing Directors publishes the ALWD Guide to Legal Citation, and the University of Chicago publishes the University of Chicago Manual of Legal Citation ("The Maroonbook").
Additionally, a number of states and courts have developed alternative legal citation systems or have different citation requirements. Legal practitioners in these jurisdictions will need to be aware of these particular requirements when they prepare and submit legal documents. Table 2 in the "Bluepages" of The Bluebook lists jurisdiction-specific citation rules and style guides.
The Bluebook, however, remains the dominant citation manual for American law.