CPM is cost-per-thousand-impressions, which is the amount an advertiser
pays each time a user views his/her ad and an impression is recorded.
As an advertiser, CPM is what you will be each time your banner reaches 1,000 impressions. As a publisher, CPM is what you will be paid each time you display a banner 1,000 times.
eCPM is the effective cost-per-thousand-impressions. This number tells you how much you are earning on a per-1,000 impressions basis. Using Adsense as an example, if you recorded 250 impressions, and have made $1.20 in clicks, your eCPM will be $4.80 because that is what you are on pace to earn over 1,000 impressions.
Exactly what eCPM is, is explained well here: http://adsense.blogspot.com/2006/02/ecpm-what-exactly-is-that.html
It is appropriate to analyze any KPI you want. Whichever you feel is the best indicator of your goals are and variables you feel you can improve.
Some common KPIs are
Cost per Click
Cost per Aquisition/Lead/Sale
Conversion Rate (Sale, Lead, etc)
Average Sale
but they could be anything that is measurable, even Revenue per Unique Visit
Browse 2000+ KPIs by business process or industry: http://kpilibrary.com/
