A cookie is a small text file that is being stored on your computer. For example, the information in the file is used to facilitate the use of the site. There are different types of cookies. A permanent cookie remains on the visitor's computer for a specified period of time. A session cookie is temporarily stored in the computer's memory while a visitor is browsing a Web page. Session disappears when you close your browser.
User who do not want to accept cookies can set their browser to automatically deny the storage of cookies or to inform the user every time a website requests to store a cookie. Through browser features, you can usually delete previously stored cookies. Read more here.