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. To earn from CPM Ads normally you need lot more traffic then for PPC or even affiliate sites. Mainly publishers use CPM ads as a secondry income or with another affiliate network. Its not easy to earn money online just by CPM Ads.
For Advertisers CPM-based advertising can be a cost-effective alternative to CPC deals. In CPC-based advertising, you pay for each click your ad receives. n a CPM-based campaign, you may pay $3 for 1,000 impressions. If your ad receives a click-through ratio of just 1 percent, you're paying $3 for the same 10 clicks. CPM ads are specailly great for Video content publishers. Traffic you receive for Streaming video sites usually don't click on ads or buy things. they simply come to watch videos So PPC and affiliate ads don't work well with these sites. Since you get paid for impressions CPM ads are the best method of earning money from this kind of sites. Most of Good CMP ad networks ask for a traffic requirement when you sign up. Still There are many who earn money online from their websites using CPM ads. One Important thing about CPM ad networks is to earn money from them you can't use just one CPM network. You need to use couple of them same time.