CPM stands for "cost per thousand impressions". Advertisers running CPM ads set their desired price per 1000 ads served, select the specific ad placements on which to show their ads and pay each time their ad appears.
For publishers, this means revenue in your account each time a CPM ad is served to your page. CPM ads compete against pay-per-click (or CPC, cost-per-click) ads in our ad auction, so only the highest performing ads will be served to your pages. Advertisers will need to bid a higher CPM than the existing CPC ads in order to show.
CPM ads can be either text or image ads and are always placement-targeted. CPM text ads will expand to take up the entire ad unit. These expanded text ads can be viewed on our Ad Formats page.