CPM advertising networks behave pretty much as PPC networks, except that you get paid according to the number of impressions (i.e., page views) that the ads displayed on your site will generate. CPM stands for Cost per Mille, and it refers to the cost for 1,000 impressions.
A blog that generates 100,000 page views monthly displaying an advertising banner with a $1 CPM, therefore, will earn $100 monthly.
CPM rates vary with the network, the position of the ad and the format. The better the network, the higher the CPM rate (because they have access to more advertisers). The closer you put the ad to the top of the page, the higher the CPM. The bigger the format (in terms of pixels), the higher the CPM.
You can get as low as $0,10 and as high as $10 per 1,000 impressions (more in some special cases). CPM advertising tends to work well on websites with a high page views per visitor ratio (e.g., online forums, magazines and so on).
List of popular CPM advertising networks: