CPM This stands for cost-per-thousand impressions. In CPM advertising, you pay for each 1000 impressions your ad receives. CPM stands for cost-per-thousand impressions. ('M' is the Roman numeral for 1000.) The price you pay is the same whether users click on the ad or not.
It depends. One important factor is where are you showinfg the ads. If it's on Google's search results, in general I would recommend CPC but if you're using Google's COntent Network, some sites have tons of traffic and a very low CPM, which would make it cheaper to go with CPM.