What are Internet Cookies?

The popular rumors about web cookies describe them as programs that can scan your hard drive and gather information about you including: passwords, credit card numbers, and a list of the software on your computer. None of this is close to the truth. A cookie is a short piece of data, not code, which is sent from a web server to a web browser when that browser visits the server's site. The cookie is stored on the user's machine, but it is not an executable program and cannot do anything to your machine.

Whenever a web browser requests a file from the web server that sent it a
cookie, the browser sends a copy of that cookie back to the server along with
the request. Thus a server sends you a cookie and you send it back whenever
you request another file from the same server. In this way, the server knows
you have visited before and can coordinate your access to different pages on
its web site. For example, an Internet shopping site uses a cookie to keep
track of which shopping basket belongs to you. A server cannot find out your
name or e-mail address, or anything about your computer using cookies.

Normally, cookies are only sent back to the server that originally sent them
to the browser and to no one else. A server can set the domain attribute for a
cookie so that any server in the same Internet subdomain as the computer that
sent the cookie will have the cookie sent along with a file request. This is
so those larger sites that utilize multiple servers can coordinate their
cookies across all the servers. The domain path can not be set to send cookies
to a subdomain outside of the subdomain where the server resides.

A cookie is sent to a browser by including a line with the following syntax in
the header of an HTML document. Note that the header is removed from the
document before the browser displays it. Thus, you will not see the header
lines if you execute the View, Source or View, Document Source commands in
your browser.