CPM
can be purchased on the basis of what it costs to show the ad to one thousand viewers (CPM). It is used in marketing as a benchmark to calculate the relative cost of an advertising campaign or an ad message in a given medium. Rather than an absolute cost, CPM estimates the cost per 1000 views of the ad.

CPA
CPA (or cost per action) is considered the optimal form of buying online advertising from a direct response advertiser's point of view. An advertiser only pays for the ad when an action has occurred.

An action can be a product being purchased, a form being filled, etc. (The desired action to be performed is determined by the advertiser.) The CPA can be determined by different factors, depending where the online advertising inventory is being purchased. CPA as Cost Per Acquisition. CPA is sometimes referred to as "Cost Per Acquisition", which has to do with the fact that most CPA offers by advertisers are about acquiring something (mostly new customers, prospects or leads). Using the term "Cost Per Acquisition" instead of "Cost Per Action" is not incorrect. It is actually more specific. "Cost Per Acquisition" is included in "Cost Per Action", but not all "Cost Per Action" offers can be referred to as "Cost Per Acquisition".