A cookie is a small text file of letters and numbers that gets put onto your computer (if you allow it) when you visit a website, meaning the site can distinguish you from other users.
Cookies are a mechanism for websites to remember things that you've done on that site in the past, which can include having clicked a particular button, logged-in, added items to a shopping cart or having read pages on that site before.
They allow you to experience the website fully, and in the case of Google Analytics cookies for example, give the website owners anonymous information to learn more about how you use and navigate around their site; meaning they can subsequently improve how the website works.
A cookie isn't a computer program, can't read files on your computer, can't carry viruses and can't install malware onto your computer.
To find out more about cookies and how you can manage them in your browser, visit www.aboutcookies.org