Often, websites need to maintain information as you move from page to page, and possibly between your browsing sessions. For example, you expect, when moving between pages in a web shop, that the site will keep track of the contents of your shopping basket. If you have logged in, you expect the site to recognise you on each subsequent page. Cookies are used to meet these expectations.
Cookies are small pieces of identifying information, stored by your browser on your PC, to enable such information about your session to be maintained. The cookie may contain actual information (e.g. the date of your last visit), or just a unique reference ID that allows the site to look up (for example) the contents of your shopping basket in its databases.
Cookies set by one site cannot be read by another. However, one web page may be made up of components (such as news stories, images or advertising), some of which are delivered from third parties. Each of those third-party sites will be able to set and read its own cookies on any page where its content appears
The site AllAboutCookies [opens in new window] provides independent information on cookies, and how to block them in specific browsers.