A cookie is a small text file which is sent from the website to your computer. It contains information about you as a user on the website to personalise your experience. For example, to determine if you are a registered user and, if so, allow you access to registration protected areas of the site or the correct user profile.
Session cookies are only stored for the duration of your visit to the website and will be removed when the browser window is closed. For example, when you close your browser window the relevant authentication cookie is removed and you will need to login again the next time you visit the website.
Permanent cookies are stored for a period of time and are used to identify you when you return to the site. For example, your username may be remembered on the login page.