CPA is an acronym for Cost per Action and refers to "ads" that are sold on the basis that money is only given when a certain action occurs because of the ad. A CPA ad could be one in which a publisher gets paid for people that click thru an ad and sign up for a newsletter or enter a contest or fill out a form for more information on a product. Many consider direct sales or Cost Per Sale ads, and CPC ads to also be a form of CPA. But CPA usually refers to being paid for leads, sign ups, and so on.