I know that eCPM stands for effective cost-per-thousand. My question is does google pay by clicks and impressions or just clicks?
I think it is the ammount the advertiser has paid per 1000 impressions. It says somewhere on the site.
They pay just for clicks. The eCpm is just for show and really has no relevance except for comparison basis. You really want to be concerned about your ctr (click through rate) and cpc (cost per click) which is total $'s earned/# of clicks. The goal is to increase both ctr and cpc to earn on Adsense. There are many legitimate ways to do both, and Adsense will actually give you good ideas to increase ctr. Increasing your cpc takes more work, and is somewhat a trial and error process, but if you're diligent it can pay dividends.