A cookie is a small piece of data that a website stores on your device when you visit, typically containing information about the website itself, a unique identifier that allows the site to recognise your web browser when you return, additional data that serves the purpose of the cookie, and the lifespan of the cookie itself.
Cookies are used to enable certain features (eg. logging in), to track site usage (eg. analytics), to store your user settings (eg. timezone, notification preferences), and to personalise your content (eg. advertising, language).
Cookies set by the website you are visiting are normally referred to as "first-party cookies", and typically only track your activity on that particular site. Cookies set by other sites and companies (ie. third parties) are called "third-party cookies", and can be used to track you on other websites that use the same third-party service.