The more common form of arbitrage is buying traffic from 2nd or 3rd tier search engines and driving them to a "Made for AdSense" (or sometimes Yahoo Publisher Network) kind of site.
Google frowns upon this practice, though. Their reaction to arbitrage is to implement "smart pricing" for the AdSense account whereby clicks generate about 1/10th the revenue compared to a real content-based site.
Both Shelly and Ghoti are telling you accurate definitions ... weird huh? Well, the meaning of arbitrage has changed over time, so it makes them both right.
You might hear about arbitrage in terms of buying cheap search engine ads. I've heard Shoemoney talk about it with regards to the current prices on MSN.
Black Hatters will go to the small search engines and buy cheap ads that will drive traffic to affiliate linked landing pages ... and like Ghoti said, some of them drive that cheaper traffic to the Made for AdSense sites. Doing this makes their costs less and profits more.
The basic meaning is cheap ads.
You might want to look at "click flipping" too. That might give you an insight into a current aspect of arbitrage.
What is arbitrage?
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