1 pixel .gif in Emails ?

I read this paragraph in a corporate Privacy Policy statement and did not understand what it means

We may access and set cookies using web beacons, also known as single-pixel GIFs which are invisible graphical images. These web beacons provide us useful information regarding our site such as which pages users access. When we send you e-mails, we may include a single-pixel GIF to allow us to determine the number of people who open our e-mails. When you click on a link in an e-mail, we record this individual response to allow us to customize our offerings to you.

Any ideas...?

So, it is just a form of tracking. Personally I wouldn't allow the e-mail, but that is just me. Check out these forums for similar types of things in peoples signatures.
Many of the members here have images that display your IP, number of times you have visited the forums etc. - that is the same thing (only a bigger picture )

Also what they can do is append your email address to the URL. So you have an image tag like this

<img src="tracker.php?email=your@address.com">

tracker.php will return a 1 pixel GIF image once its done whatever it needs to do.

I appreciate both responses to this thread. It sounds like a concept that may be of use to me at some point.

However, I am still confused (newbie here). It's just not registering with me what is taking place.

...I receive an email, open it, and it has a .gif somewhere that I can't see. Then there's an image tag that has no image but a php script instead.

Doesn't there need to be an <a> tag to click somewhere, otherwise how does the sender know if the email was opened?

Pardon my density.

In the script there is probably some code that just recognizes that the script is being called. The script (or gif) is called unless you open up the e-mail and view it.

The code in the script will first log the details from the link, and then send a transparent gif image to the users page. This allows the sender to log who has looked at the email.

If I use the php function mail($to, $subject, $message), then do I need to include HTML in the $message ... <img src="http://www.mysite.com/tracker.php?email=$to">

Then does tracker.php use a header('Content-type:image/gif') call before issuing readfile(images/1pixel.gif) to send the image ??