IRVINE, Calif.—David Norris wants to collect the digital equivalent of fingerprints from every computer, cellphone and TV set-top box in the world.

He's off to a good start. So far, Mr. Norris's start-up company, BlueCava Inc., has identified 200 million devices. By the end of next year, BlueCava says it expects to have cataloged one billion of the world's estimated 10 billion devices.

Device fingerprinting is a powerful emerging tool in this trade. It's "the next generation of online advertising," Mr. Norris says.

It might seem that one computer is pretty much like any other. Far from it: Each has a different clock setting, different fonts, different software and many other characteristics that make it unique. Every time a typical computer goes online, it broadcasts hundreds of such details as a calling card to other computers it communicates with. Tracking companies can use this data to uniquely identify computers, cellphones and other devices, and then build profiles of the people who use them.