Cost per acquisition (CPA), also known as pay per acquisition (PPA) and cost per conversion, is an online advertising pricing model where the advertiser pays for each specified acquisition - for example, an impression, click, form submit.
CPA i good only for certain niches. I much prefer working with CPM networks. Currently working with Ad Maven and my earning much more stable than what they used to be with CPA.
CPA means cost per acquisition, it is a kind of bid strategy in which chance of coversion get increases when your ad gets clicked. In this strategy after getting your conversion history, Google will control your ads impression to the user who are more liable to conversion.