eCPM stands for effective cost per Mille (or effective cost per thousand impressions) and RPM stands for Revenue per thousand impressions. Both the terms usually mean the same but they are used in different contexts.
When advertisers want to run display ads on a particular website, they will calculate the eCPM and see if they can afford to run the ad. Advertisers will compare the eCPMs of various channels and determine which one is converting better. The obvious choice would be to go with the channel which offers the least eCPM paired with maximum conversions.
RPM is usually publisher’s language and they find out how their web property is performing. Page views keep changing but RPM for a website in a specific industry and with a specific audience fairly remains constant over a period of 6 months to a year. A health niche website may have a higher RPM and a cine gossip entertainment website may have a relatively low RPM compared to the health website.