Web beacons, clear GIFs, page tags and web bugs
These are all terms used to describe a particular form of technology implemented by many sites in order to help them to understand and analyse how their site is being used and, in turn, to improve your experience of the site. They may also be used to target any advertising being served on the web page you are viewing.
A web beacon (or similar) usually takes the form of a small, transparent image, which is embedded in a web page or an email. They are used in conjunction with cookies and send information such as your IP address, when the page or email was viewed, from what device and which geo-location etc.
As a web beacon forms part of a web page, it is not possible to ‘opt-out’ as such but you can render them ineffective by opting out of the cookies they set. Manage your cookies.