From facebook info:
Pixel tags (also called clear GIFs, web beacons, or pixels) are small blocks of code on a webpage that allow websites to do things like read and place cookies. The resulting connection can include information such as the person’s IP address, the time the person viewed the pixel and the type of browser being used.
We use pixels on and off Facebook, like when you visit our site or one of our partners. Pixels allow us to read any existing Facebook cookies or also place a new cookie on your browser or device. We use pixel tags to customize your experience and learn about how people use products and services. For example, we can use pixel tags to see that a person using a certain browser saw an ad on Facebook and also bought a product from that advertiser. This helps us show advertisers that the ads they run on Facebook are effective. We also may use pixels to help show you an ad on or off Facebook. For example, a partner may use a pixel to tell us when you’ve visited its site so that we can later show you an ad on Facebook.
I am none the wiser, has anyone got examples or can demonstrate how ths technology works ?