Shepardize: To determine the subsequent history and treatment of (a case) by using a printed or computerized version of Shepard's Citations. Also, to check the precedential value of (a case) by means of shepardization.
--Black's Law Dictionary, Ninth Edition
Shepardizing: "... to see if a case has been overturned, reaffirmed, questioned, or cited by later cases."
Why is it important to Shepardize a court case? The American legal system is based on stare decisis, which is the "doctrine of precedent, under which a court must follow earlier judicial decisions when the same points arise again in litigation" [Black's Law Dictionary, Ninth Edition]. Shepardizing determines if a particular court case has been overturned or overruled or reversed by a higher court, and thus becomes null and void.
NOTE: The Westlaw database employs a similar citator tool as Shepard's Citations called KeyCite. From the home screen of Westlaw, open the "Campus Help Guide" near the bottom of the screen for directions in using KeyCite