Page RPM literally means Page Revenue Per Thousand impressions. The letter "M" in the abbreviation is the Roman numeral which means one thousand (alternatively it can be "mille" which means thousand in Latin). Page RPM is basically a calculation which shows the average projected revenue of a page per thousand impressions that it gets.
Page RPM is a useful figure for comparing the performance of different webpages or even advertising networks. By scaling the earnings of a page to what would happen if the page got a thousand impressions, we are able to compare pages independent of the number of impressions that each particular page got, and independent of the advertising network.
Page RPM can be calculated easily using the following formula:
Rather than focusing on things such a click-through-rate and cost-per-click that may vary greatly, we instead focus on a single figure which describes the projected earnings for 1000 views of a page or pages on a website.
For example, if you earned $0.25 from 78 page views, then your page RPM would be equal to ($0.25 / 78) * 1000, which is $3.21.
Without a doubt, if you are a publisher you would want your Page RPM to be as high as possible.
CPM (which means Cost Per Mille) is an indication of what it will cost the advertiser if their advertisements are shown to 1000 viewers. It is a metric similar to RPM, but CPM is useful to the advertiser, while RPM is useful to the publisher or person who will be showing the advertisements.
RPM for the publisher will always be less than or equal to the CPM of the advertiser. This is because miscellaneous charges and overheads may be taken out (of what was originally paid by the advertiser) by the advertising networks and other intermediaries.
The RPM is therefore the figure which is interesting to the publisher.