CPA marketing is easily defined as a method through which a company pays you for promoting their offers online; some of such offers could be filling a simple form, making a purchase or completing a survey, entering your email. Companies especially those in the United States will gladly pay a publisher/ affiliate that promotes their offers.
By promoting offers, you will simply be referring someone to take an action using the tracking link provided to you by the CPA network.
The interesting thing about CPA marketing is that the people you refer do not necessarily have to make a purchase for you to get paid. That is what I usually call easy money.