A web bug/beacons is an object that is embedded in a web page or email and is usually invisible to the user but allows checking that a user has viewed the page or email.[1] Common uses are email tracking and page tagging for Web analytics. Alternative names are web beacon, tracking bug, tag, or page tag. Common names for web bugs implemented through an embedded image include tracking pixel, pixel tag, 1×1 gif, and clear gif.[2] When implemented using JavaScript, they may be called JavaScript tags.

Lets see an example :(To Check whether the mail recipients has read the mail or not)
Now-a-days an email is written in HTML Format.So you can insert the below code easily.

What will happen here is,the Email client will try to fetch the image from the src path, But instead of actual image the PHP script will get executed.So this way you can check if the user has read the mail or not.

Note : In most of the email clients images are blocked by default,in that case this will not help.

Ref:
Wiki