The website stores a corresponding file(with same ID tag)to the one they set in your browser and in this file they can track and keep information on your movements within the site and any information you may have voluntarily given while visiting the website, such as email address.
Cookies usually don't contain much information except for the url of the website that created the cookie, the duration of the cookie's abilities and effects, and a random number. Due to the little amount of information a cookie contains, it usually cannot be used to reveal your identity or personally identifying information.
You have the ability to accept or decline cookies by modifying the settings in your browser. For more information visit www.aboutcookies.org which contains comprehensive information on how to modify the cookie settings on a wide variety of browsers. You will also find details on how to delete cookies from your computer as well as more general information about cookies.