Magic cookie

A magic cookie, or just cookie for short, is a
token or short packet of data passed between
communicating programs, where the data is
typically not meaningful to the recipient
program. The contents are opaque and not
usually interpreted until the recipient passes
the cookie data back to the sender or
perhaps another program at a later time. The
cookie is often used like a ticket – to identify
a particular event or transaction
HTTP cookie
A cookie, also known as an HTTP cookie, web
cookie, or browser cookie, is a small piece of
data sent from a website and stored in a
user's web browser while the user is
browsing that website. Every time the user
loads the website, the browser sends the
cookie back to the server to notify the
website of the user's previous activity
Cookies were designed to be a reliable
mechanism for websites to remember
stateful information (such as items in a
shopping cart) or to record the user's
browsing activity (including clicking
particular buttons, logging in, or recording
which pages were visited by the user as far
back as months or years ago).