A cookie is a small amount of data that servers send to the browser in the HTTP protocol. The cookie is stored on the user's computer.
The browser then sends this data back to the server each time the user visits the site. Cookies are commonly used to distinguish individual users, to store the contents of "shopping baskets" in electronic shops, user preferences, etc.
Technically, they are a series of codes by which the browser collects and then sends information about our behaviour back to the relevant server.
Cookies contain information about how long the browser should keep them (from one-off to several years).
Cookies are not directly linked to our name or email address, but often contain our login details. They are common to a single browser installation. Cookies are not secure in any way and this includes the data collected.