Cost per action (CPA). In online advertising, is a method used to put a price on a given action of a visitor. The advertiser pays a specific amount for the action to perform when a visitor arrives through an online advertisement. This action may be a purchase, signup, etc.. The shares to be paid are decided by who announced, while the amount payable may be decided by the advertiser, by an arrangement between the parties, by auction among other ads, etc..
Every action has a price, which can be for example a percentage of sales achieved thanks to a poster advertisement. The advertiser pays for every action a visitor takes effect, thus obtaining a guaranteed success for the money.