Usually when people are babbling about CPA on the boards they're talking about things like zip or email submits, where you get paid (usually something like 0.50-$3) per email address or whatever you can get a surfer to give to an advertiser. I don't know if I'm allowed to post links here to affiliate networks that have CPA offers, so I won't; however, if you want to know more, PM me.
CPA (Cost Per Action) means that publisher recieve money for every action (lead or sale) made by users on advertiser site. Sometimes it's called PPS – pay per sale or PPL – pay per lead.
CPA is mostly used by low quality spammers and companies trying to get cheap leads - but ultimately being ripped off by blackhat seo spammers!
A CPA or certified public accountant: an accountant who has passed certain examinations and met all other statutory and licensing requirements of a United States state to be certified by that state; "in addition to accounting and auditing, CPAs also prepare tax returns for individuals and corporations"