What is Traffic Acquisition Costs?

[cache]

I am reading the news here:

http://www.forbes.com/2006/01/30/google-ebay-yahoo-0130markets08.html?partner=yahootix

what is an example of Traffic Acquisition Costs? I thought it does not cost google anything, it only costs the publisher to bring the traffic to their sites?

 

[Smith]

Google pays people who run websites and park domains using their feeds a % of what they make per click.

 

[JMJ]

Such as the payment you receive from the adsense ads on your sites.