The New York Times > Business > Your Money > What Wal-Mart Knows About Customers' Habits: "Plenty of retailers collect data about their stores and their shoppers, and many use the information to try to improve sales. Target Stores, for example, introduced a branded Visa card in 2001 and has used it, along with an arsenal of gadgetry, to gather data ever since. But Wal-Mart amasses more data about the products it sells and its shoppers' buying habits than anyone else, so much so that some privacy advocates worry about potential for abuse.
With 3,600 stores in the United States and roughly 100 million customers walking through the doors each week, Wal-Mart has access to information about a broad slice of America - from individual Social Security and driver's license numbers to geographic proclivities for Mallomars, or lipsticks, or jugs of antifreeze. The data are gathered item by item at the checkout aisle, then recorded, mapped and updated by store, by state, by region.
By its own count, Wal-Mart has 460 terabytes of data stored on Teradata mainframes, made by NCR, at its Bentonville headquarters. To put that in perspective, the Internet has less than half as much data, according to experts."