A cookie is a small amount of data which often includes an anonymous unique identifier
that is sent to your browser from a website's computer and stored on your computer's hard drive.
Each website can send its own cookie to your browser if your browser's preferences allow it,
but (to protect your privacy) your browser only permits a website to access the cookies it
has already sent to you, not the cookies sent to you by other sites.
Many sites do this whenever a user visits their website in order to track online
traffic patterns and record information about your online preferences.