Nice but short article on data mining at Acxiom.
These data miners know everything about you - Acxiom (1) - CNNMoney.com
"Who they are: The oldest and the biggest data aggregator out there, Acxiom says it has more than 32 billion data records. If you get direct-marketing mail, phone calls from telemarketers, offers by e-mail or ads targeted to you online, there's a pretty good chance the company behind it looked up information about you on Acxiom."
"What they know: Pretty much everything. The company tracks all kinds of personal information, from your Web behavior to your Social Security number and your finances."
The Wikipedia article has a little more info:
Acxiom - Wikipedia, the free encyclopedia