A cookie (sometimes known as web cookie, browser cookie or html cookie) is a small amount of data (<4Kb) that is sent to your computer or web-enabled device (hereafter referred to as a "device") browser from a website's server. The cookie can include a unique identifier within it.
This website sets three cookies:
Essential cookies
"_" - this is a transient (session) cookie which is used to identify the logged in user - to allow them to complete census returns and view data within their part of the hierarchy. (This is a session cookie so is deleted when you shutdown your browser.)
Functional cookies
The cookie is stored on your device's hard drive or (volatile) memory (this is often called "setting a cookie"). Each website that wishes to send you its cookie can only do so if your device's web browser preferences, that you can set personally, are set to allow this. To protect your privacy your device browser only permits a website to access the cookies it has sent to you, not the cookies sent to you by other websites. Many websites set one or more cookies to track online traffic through their website.
This site sets a cookie to store information about your access rights i.e. your ability to login to restricted services and the duration of that login
Users can set their device browsers to accept all cookies, to notify them when a cookie is issued, or not to receive cookies at any time. The last of these means that restricted access areas of the website would be unavailable to them. Each browser is different, so check the "Help" menu of your browser to learn how to change your cookie preferences.