Recently, Mozilla Firefox launched a new add-on called Collusion (download via link). What it does is that it creates a web of the sites you have visited and connect them to other websites that tracks your information from the sites you have visited. When you think about about it, it is a bit shocking knowing that in a spam of a few hours, you have over 100 sites tracking your web activity without you noticing.
I am still new to this concept. Hopefully Gary Kovacs, CEO of Mozilla Co, can explain this better through this video:
In the coarse of one afternoon, after spending about a couple of hours doing what I usually do on the internet, I got close to 50 unknown sites tracking my online activity. Here is my graph
If you are interested in knowing who is tracking you, try Collusion