A cookie is a small text file that is sent by a website to your computer or mobile device where it is stored by your web browser. A cookie contains limited non-personal data, usually a unique identifier and the name of the site. This enables a website to recognise you as you move around the site and/or each time you revisit. Cookies are used for a wide variety of purposes such as to keep you logged in or to remember what's in your basket if you're shopping online, to remember your preferences and settings, to analyse how the site is used by you, and to serve advertising to you.
Cookies may be served to you by the website you are visiting (a "first party cookie") or by another organisation providing services to that website, such as an analytics company or advertising network (a "third party cookie"). They will either be stored for the duration of your visit (a "session cookie") or they will remain on your device for a fixed period, which could be months or even years, to remember you across multiple browsing sessions (a "persistent cookie").