A service that offers online ad space for sale to advertisers. This space can represent inventory from hundreds and thousands of websites. A general rule is that some inventory is more valuable than others, so it costs advertisers more. Think of buying a TV ad placement during “the big game” versus during a mop infomercial at 3 am. Depending on the ad network, the payment structure may be based on cost per thousand impressions (CPM), cost per click (CPC), or cost per acquisition (CPA).