information placed on a visitor's computer by a web server. While the website is being accessed by the visitor, data in the visitor's cookie file can be stored or retrieved. Mostly cookies are used as unique identifiers (i.e. user IDs or session IDs) to isolate a visitor's movements from others' during that visit and subsequent visits. A cookie is a small file stored by the visitor's browser and read by the site to remember who the visitor is. Cookies are commonly used to measure visitors. Using cookies as a traffic measure actually measures the computer and not the person because cookies are computer-based. Therefore, all users sharing the same computer have the same cookie and will be aggregated together as a single visitor.