CPM is cost per thousand impression ad. Advertisers running CPM ads set their desired price per 1000 ads . You will get paid whenever there are 1000 impressions for that CPM ad. CPM ads compete against CPC ads in Google's ad auction system, so only the highest performing ads will be served to your pages. Both image ads and text ads can be paid on CPM basis, and both of them are always site-targeted. One thing that you should always bear in mind is that, for CPM ads, your earnings are associated with impressions only, rather than clicks. Google pays you each time a CPM ad is shown on your page, rather than when a user clicks on the ad and views the advertiser's site.
Actually, the statistics (ad impression and clicks) of CPM ads will be shown together with that of CPC ads in general report in your account. Sometimes, you may notice that even though you've received impressions for CPM ads, you're not displaying any earnings. No problem. This is because an advertiser may pay as little for each 1000 impressions of an ad. So, you may afraid your earning will be lost when it cannot come to a significant figure. No worry. Although Google will only show earning that is more than one cent, all the earning from impressions of CPM ads that is below one cents will be recorded and accumulated. It will then show whenever it reaches one cent.
Since CPM ads are paid on the number of ad impression, the location of the CPM ads highly affect the overall earnings for you. For example, if you put your CPM ad on top of your site, you definitely recieve more ad impression for that and have higher income, and vice versa.
Although there is no offical figure to prove, there is a general belief that most of the image ads are paid on CPM basis, while only little text ads are paid on CPM basis. To be honest, from my experience, CPC ads perform a lot better than CPM ads. Suitable place to put CPM ads is forum site, it is because in a forum site, the usual and regular visitors already get used to the ads there. They tend to have a much lower CTR, and thus CPM ads (which are not paid on clicks) are the most effective to maximize your earning.
