A cookie is a small text file that stores Internet settings. Almost every website uses cookie technology. The cookie is downloaded by your Internet browser the first time you visit a website. The next time you visit this website from the same device, the cookie and the information in it are either sent back to the originating website (first-party cookies) or to another website to which it belongs (third-party cookies). By that, the website can detect that it has already been opened using this browser, and in some cases, it will then vary the content it shows. 

Some cookies are extremely useful because they can improve your user experience when you return to a website you have already visited. This assumes that you are using the same device and the same browser as before; if so, cookies will remember your preferences, will know how you use the website, and will adapt the content you are shown so that it is more relevant to your personal interests and needs.