The law is constantly changing with new cases being decided everyday, so making sure that the cases that you cite in your research are still good law is very important. You also need to understand how the points of law in your case are being treated by other courts. Citators help you do this by showing the history of a case, and how it has been treated in subsequent decisions. Indicators such as red flags and stop signs are used to help identify points of law that may have received negative treatment from higher courts.
Citators have the added benefit of helping locate secondary sources, such as encyclopedias, treatises, and law review articles, that can enhance your understanding of the case and further your research into a topic.
Lexis Advance, WestlawNext, and Bloomberg Law each offer their own citator tools. Lexis offers Shepard's and Westlaw offers KeyCite. Bloomberg offers a program called BCite. Check out some of the links on this page for more information detailing how to use each one.
KeyCite is available through WestlawNext, and can be used to locate the history and citing references for cases, as well as statutes, regulations, and administrative decisions.
Checking Citations in KeyCite: a handy guide to using KeyCite.
Using KeyCite to Verify and Expand Your Research: a brief video covering some of KeyCite's features.
Shepard's is the citator service used in Lexis Advance. It also provides access to case history and subsequent treatment. "Shepardizing" has become shorthand for checking to see if a case is still good law.