A hard disk functions somewhat like a record player. Just as a needle reads the intricate pits and grooves on the surface of a record, a slider reads the combinations of 0s and 1s on the surface of a hard disk. These strings of digits translate into your English essay, MP3 file or even your most recent report card. Data is stored on a hard disk in tracks and sectors. Tracks are concentric circles like tree rings.
Sectors are pie-shaped wedges.
Picture a series of concentric rings and wedges, with a fixed set of data space within each series. The slider reads various tracks and sectors at lightning speed to decode 0s and 1s into information. Your name can just be your name and address passed to other companies, but your profile may contain info on your age, ethnicity, spending patterns and health status. Sending out questionnaires and surveys is slow and costly and often results in incomplete data. But following you as you surf the Web, make ATM charges and order groceries online and compiling data to build a profile is fast, cheap, accurate and perfectly legal.