From the Google Adsense help:
I didn't know that. I've never noticed any earnings from CPM ads, perhaps because the earnings are small in comparison to CPC (cost per click) ads.CPM stands for "cost per 1000 impressions." Advertisers running CPM ads set their desired price per 1000 ads served and pay each time their ad appears.
As a publisher, you'll earn revenue each time a CPM ad is served to your page and viewed by a user. CPM ads compete against cost per click (CPC) ads in our ad auction, and we'll display whichever ad is expected to earn more revenue for you.