How does google Cost Per Acquisition work?

How does google Cost Per Acquisition work? Has any one tried it? ANSWER TO THE ABOVE QUESTION Google Cost per Acquisition or Cost Per Action(CPA) is a advertising system that allows one to pay only when a conversion happens(like actual sale, leads, download, etc.) as opposed to CPC(Cost Per Click) system in which one needs to pay each time someone clicks on a PPC ad.CPA is a good way to control advertising spend and enhance ROI