A cookie is a small piece of text sent to your browser by a website you visit. It contains information about your visit that you may want the site to remember, like your preferred language and other settings. The browser stores this data and pulls it out the next time you visit the site to make the next trip easier and more personalized. If you visit a movie website and indicate that you’re most interested in comedies, for instance, the cookies sent by the website can remember this so you may see comedies displayed at the start of your next visit.
Some people prefer not to allow cookies, which is why most modern browsers give you the ability to manage cookies to suit your tastes. You can set up rules to manage cookies on a site-by-site basis, giving you greater control over your privacy. What this means is that you can choose which sites you trust and allow cookies only for those sites, blocking cookies from everyone else. Since there are many types of cookies ”” including “session-only cookies” that last only for a particular browsing session, or permanent cookies that last for multiple sessions ”” modern browsers typically give you fine-tuned controls so that you can specify your preferences for different types of cookies, such as accepting permanent cookies as session-only.
Definition courtesy of Google Chrome’s “20 Things I Learned About Browsers and the Web”