Cookies are small pieces of data that are downloaded to a user's computer or mobile device when you visit a website.  Your browser sends these cookies back to the website every time you visit the site again, so it can recognise you and can then tailor what you see on the screen. A cookie often includes a randomly generated number which is stored on your device and many cookies are automatically deleted after you finish using the website.  They make using websites much smoother and affect lots of the useful features of websites.