A small file, a specific purpose
A cookie is a small text file stored on your device when you visit a website. It allows the site to remember your device between pages or visits, for example to keep a form working or to record that you have already seen a notice.
Cookies do not run programs and do not carry viruses. They are passive strings of text — not applications — and they are read only by the domain that placed them or by a provider that domain has authorised.