Cookies are sent from web servers and are stored by client web browser applications. They are used to allow web servers to know what activities the client in question has performed when interacting with the server. Cookies can be session based, or as in the case of Persistent Cookies, can be retained by the browser between web sessions. Persistent Cookies have a Max-Age attribute that can be used to specify how long they are to be retained by the client web browser. Persistent Cookies are often used to track the websites that a client visits, and which links between sites they follow. For this reason they are often referred to as Tracking Cookies.