Metrics in digital marketing

There are quite a few metrics in digital marketing. Some of them are easy to understand. Some are not.

Here you'll find definitions of both the easy ones and the not so easy ones.

Definitions

What is really the difference between CPM and eCPM? And is really CPM a metric or a payment model in digital marketing?
Buying methods explained
Definition of CPM
Definition of eCPM

Calculation methods

And how do we calculate these metrics?
How to calculate CPM
How to calculate eCPM

I also recommended using the CPM calculator to make sure you get your numbers right.