Placing ads on a site
 

I've looked at the code of the first vertical banner ad on the main page of sex.com, and it looks something like this:

I've seen a lot of xxx websites (especially the big / high traffic ones) doing something like this, using different ulr for the ads.

Do you know / can you tell me how this works?

Thank you.

Is it possible to track number of clicks, are where they are coming from (number of clicks per referring domain would be the best; the more info the better) with something like this too?