Basics of Digital Advertising

CPM (Cost per Mille / Cost per Thousand impressions):
This represents the number of time your Ad appears on the publisher’s platform. Here the cost is calculated once your ad gets visible to minimum 1000 people & then it gets calculated every time 1000 ad impression is visible.
CPC (Cost per click):
Here advertiser is charged every time someone clicks on his ad.
CPA (Cost Per Acquisition / Cost Per Action):
Here advertiser is charged every time an action occurs or every time an Acquisition of customer occures.
CPL (Cost Per Lead):
Here advertiser is bound to pay every time the user fills the Lead form. Though u will not find this as a cost matrix in Google AdWords directly. This term will be used by marketers heavily in their media plans.