HTTP cookie
When someone is using a computer to browse a website, a personalised cookie file can be sent from the website’s server to the person’s computer.  The cookie is stored in the web browser on the person’s computer. At some time in the future, the person may browse that website again. The website can send a message to the person’s browser, asking if a cookie from the website is already stored in the browser.  If a cookie is found, then the data that was stored in the cookie before can be used by the website to tell the website about the person’s previous activity.  Some examples where cookies are used include shopping carts, automatic login and remembering which advertisements have already been shown.