Many of the advertisements people see online today are customized. Using so-called browser cookies, advertisers can track a given Web surfers' habits and serve them relevant ads.

This election year, a related type of targeted ads—one relying on "political cookies"—is coming into widespread use.

The technology involves matching a person's Web identity with information gathered about that person offline, including his or her party registration, voting history, charitable donations, address, age, and even hobbies.