CPC vs CPM

CPC means Cost per Click Ads, while CPM means Cost-Per-Thousand Impressions of the Ad.

The graphical announcements and text announcements can be paid as much by CPM as by CPC. Google decides automatically wich one to display according the bids of each format of ad for a especific site according the contextual keywords that surround the Adsense code.
So the publisher cannot decide wich type of Ad to show in his site, he just put the code and Google deliver either CPC or CPM ads.