A citator is a service that shows you:
"Updating cases" means checking a citator to see if the case has been overturned, making it bad law. Each of the three big research services-- Lexis, Westlaw, and BloombergLaw-- has a citator, making updating your cases quick and simple.
Use the videos at the right to familiarize yourself with Shepards (Lexis), Keycite (Westlaw), and BCite (BloombergLaw).