It's that multiple sites will serve content (ads, Javascript libraries, like buttons) from a common site (eg an ad network) that uses its own domain. That domain is allowed to get the cookie for itself because it is referenced by multiple site, that's how this type of tracking works.
If you go to bbc.com, it still won't be able to see cookies from cnn.com, but say if advert.com is included by both sites, then it will see that you visited them both. That's the power and great danger that things like facebook and google sense represent.
The owner of the sites get some stats for free by using these services, but the biggest benefit is for google and facebook to be able to track what users are looking at accross the web. And you just need to be identifiable on one site that you visit (say, FB or gmail) for them to know exactly who you are.
From what I understand, Firefox will only allow advert.com to get the cookie it created when being loaded as part of bbc.com, but it won't be able to read its own cookie from cnn.com, it will have to create a new, separate one, thus breaking the link tracking you between sites, or at least making it harder to connect the dots.
Everyone should use FF.