CPM stands for cost per mille. CPM is the amount an advertiser pays to have its ads served against videos 1,000 times. Each time when an ad on a video runs completely, it is called an impression.
eCPM means effective cost per mille. It is the outcome of a calculation of the ad revenue generated by a banner or campaign, divided by the number of ad impressions of that banner or campaign expressed in units of 1,000.