CPA stands for cost-per-action. CPA essentially measures actions that are taken by the web site visitor that result in a purchase or an equivalent action such as signing up for a newsletter. An online marketer is relieved of some risk and is giving their advertisement investment a better value as this model places a lot of the responsibility of conversion to the publisher of the web site. Think of it in terms of earning commission. The goal of the online marketer is to compose a creative campaign that is capable of generating actionable leads and pays for each lead which results in some action being taken.