It is the amount an advertiser pays per 1,000 ad impressions.
if you purchase 1,000,000 ad impressions at $2 CPM you would pay $2,000 for it. This is because for every $2 you spend, you are getting 1,000 ad impressions.
Often referred to as PPC (pay per click)
It is the amount paid every time a user clicks on an ad.
If you purchase 1,000 clicks for a $10 CPC, then you owe $10,000 if they are all delivered.
It is the amount that gets paid for every day that an ad is on a webpsite for the whole day.
If you purchase $50 CPD for 100 days on the website sport page, then you will pay $5,000 in total and receive every ad on the sport page for 10 days.