Difference between CPC and CPM ads:

This is a popular FAQ: I have got 2500 page impressions of CPM ads, but still haven't earned a cent.Why?
The reason is that the Ads which you think are CPM( Cost Per 1000 impressions) ads, are not so.You can not differentiate between CPC ads and CPM ads.

Let's try to analyse the two:
1.CPC ads:CPC( Cost Per Click) ads are very easy to understand.When a visitor clicks this type of ad, you get the revenue depending upon the advertiser.These earnings per click may vary between $0.03 to $10.00 (it depends upon the topic of your blog and the will of advertiser).Usually, the text ads are the CPC ads.

2.CPM ads:CPM (Cost Per 1000 impressions) ads are a bit different from that of the CPC ads.In CPM ads, the advertiser pays you some money per 1000 impressions.But the problem with these ads is that we can not surely tell that the ad xyz is CPM ad.So, it is possible that you may have not earned anything with 10,000 page impressions (if no click is generated).
The ad being displayed as CPM ad, may change to CPC ad irrespective of any change from your side.
Generally, the image ads are served on CPM basis.

Let's say that the advertiser is paying $0.30 per 1000 impressions.
1000 impressions = $0.30
1 impression = $0.0003
When you get 10 impressions of CPM ads, you will earn $0.0003 x 10 =$0.003.
According to Google Adsense, they show the earnings only if they are more than $0.01 ( hence, the $0.003 figure will not be shown in your account.).If you get 100 CPM impressions, you will earn $0.03 and this will be displayed in your account.

It is better to go for the CPC ads because the earnings are very much higher.
But if you have a website getting as more as 2,00,000 page impressions per day and very less number of clicks( and also very low CPC), it is better to use CPM ads.