What They Know: A Glossary

Key tracking terminology

Ad exchange -- An auction-based marketplace where advertisers can bid to place ads in the space offered by websites.

Ad network -- A company that sells ads on behalf of website publishers.

Aggregated information -- Data combined from many individual users that can't identify anyone personally.

Anonymous information -- Facts about you that don't identify you personally, such as age group and gender.

Beacons -- Invisible software on many websites (also known as "bugs" or "pixels") that can track web surfers' location and activities online. Some are powerful enough to know what a user types on a particular site.

Behavioral targeting -- Advertisers and websites use information about where you browse and what you search for online to guess your interests and decide what ads to show you. It's also called interest-based advertising or customized ads.

Cookie -- Tiny text file put on your PC by websites or marketing firms that—depending on its purpose—might be used simply to remember your preferences for one site, or to track you across many sites.

Data exchange -- A marketplace where advertisers bid for access to data about customers. Marketers then use this data to target ads. For example: A Denver hotel might bid to reach people known to have researched Denver hotels recently.

Exposure index -- The Journal's analysis of how exposed your data is when you visit a website that has trackers. Each tracker was given a score based on how the tracking company collects, shares, and uses your data. A website's exposure index was calculated using the sum of the scores of all of the trackers we found on that site.

First-party tracking file -- Typically a cookie installed on your computer by a website for benign purposes such as keeping you logged in to that one site.

Flash cookie -- Small file put on your computer by Adobe's Flash software, which is used by many sites to display video or ads. Flash cookies can be designed to re-install regular cookies that were previously deleted.

Internet Protocol (IP) address -- A unique number assigned to every computer connected to the internet. Any website you visit can know your IP address, and through that can often know your general location.

Offline data -- Information about you that comes from sources other than the Internet. It could include your zip code, estimated household income, the cars you own, or the purchases you've made in a store.

Personally identifiable information -- Data identifying you uniquely, such as your name, Social Security number, address or credit-card information.

Privacy policy -- A notice on a website that discloses some or all the ways the site collects or uses information.

Third-party tracking file -- A cookie, beacon or other tracking technology installed on your computer by an ad network or research firm that can track your activities across many websites.

Tracking company -- Companies that use cookies and other tracking technology to collect online data about you.

User profile -- Information about your actions, interests and characteristics that tracking companies compile about you.