The following are the top metrics used to buy online advertising:

CPM (Cost per Thousand). Buying display advertising based on how many times the ad is presented on a web page

CPA (Cost per Acquisition) & CPL (Cost per Lead). Paying advertiser only for a desired outcome like a product purchase or filed out registration form

CPC (Cost per Click). Paying advertiser for each individual click on an ad

Additional terms used:

Impressions: The number of times an ad is displayed on a web site.

View Through: When an individual is exposed to an ad but doesn’t click on it, but later goes to your web site.

ROS (Run of Site): Buying advertising anywhere on a web site as apposed to a specific web page.

CTR (Click Through Rate). Marketers optimize CPM creative to maximize CTR