While entering billing information I am getting this message - Minimum cost per thousand impressions (CPM): $0.25 I heard that Adwords charges on PPC basis but here it shows charges for CPM. What is this?
You have the choice of CPC (cost per click) or CPM (cost per thousand impression) when using the content network. Many reports also show both for comparison purposes I suppose.
CPM has very little place in the life of the average blogger/webmaster, unless you're lucky enough to be on the receiving end of thousands upon thousands of visitors a day then it's not worth your time.
I would not recommend using CPM. People tend to bid really low. You will do better on a CPC basis. Good luck!