In the media and advertising word, specifically digital, a CPA campaign is cost-per-action. Sometimes, this is also known as a cost-per-acquisition.
With this type of media buy, and advertiser is agreeing to pay a certain fee based on the number of desired outcomes.
The most common type is a direct sale. For example, an advertiser is willing to pay a media seller $1.00 for every confirmed online sale that a publisher drives. If the publisher gets 5,000 users to click an ad, and 10 of them actually end up buying the item advertised, the publisher would expect to receive $10.00 from the advertiser.
The publisher would typically not receive any monetary compensation for the 4,990 users that clicked the ad. They are only compensated for the users that completed a purchase.
Other "actions" that are typical of CPA campaigns include:
Sign ups for more information
Subscriptions to email news letters
Social media "follows" and "likes"
Phone calls made to the advertiser
Items placed into an online shopping cart