Advertisers can buy internet ad space with this service. This area can have inventory from tens of thousands of different websites. As a general rule, some inventory is more valuable than others, thus marketers pay more for it. Consider the difference between buying a TV ad during “the big game” and a mop infomercial at 3 a.m. The payment structure may be based on cost per thousand impressions (CPM), cost per click (CPC), or cost per acquisition, depending on the ad network (CPA).