CPM stands for "cost per thousand impressions".

For example, an advertiser might pay $5 for every 1,000 times an advertisement is displayed on a web site.

You need an awful lot of visitors to make good money from CPM advertisements.

You can find CPM advertising offers at some of the affiliate networks in my affiliate networks list.

For a variety of reasons, if you run CPM advertisements on your site, ad tracking software you use will often register a number of impressions that's different from the number reported by the advertising network.