From Affiliate Wiki
CPA
CPA has two meanings, one within the wider advertising world and one more unique to affiliate marketing - these can be described as
Cost Per Action
Cost-Per-Action is a way of providing a cost effective advertising solution to a Merchant, where the advertiser is paid only on the result of a purchase or equivalent action which may include signing up to a mailing list or registering as a user on a website.
Cost Per Acquisition
this is the total sum spent either via paid search, email marketing or banner advertising to generate a valid sale
