A savvy computer user may block websites from depositing small data files, called "cookies," on their computer for privacy reasons. On the other hand, cookies are also the technology used by many companies to note when a user opts out of targeted advertising.
Websites have put cookies onto visitors' computers to track usage for nearly two decades. They can keep users logged in to their accounts, track preferences or identify repeat visitors. For example, the little "Remember me" check mark when you log into Facebook deposits a cookie.
Cookies are discoverable inside a folder on any connected computer's hard drive, but they can be blocked within the options of many web browsers.
In Firefox, for example, the controls for cookies can be accessed in the options menu, by choosing to "use custom settings for history." In Chrome, the option is buried in under the "Advanced Settings" menu, which is a subset of the "Content Settings" menu. In Internet Explorer, cookies can be blocked within the "Advanced Privacy Settings" in the "Internet Options" menu.
But there is a consequence to blocking cookies worth considering.
Cookies are also the tool many sites and services use to record preferences for blocking targeted ad services.
The Digital Advertising Alliance, which includes Google and 98 other companies, has an online web tool for opting out of ads targeted because of your past behavior online. The DAA offers a web form that allows users to opt out of some or all of the participating services, but it deposits a cookie to record that decision.
Therefore, blocking cookies will not necessarily prevent ads from being targeted based on your past behavior online and blocking targeted ads requires you to allow tracking through cookies.