An Insertion Order (IO) is a formal agreement between an advertiser and a publisher or media company, outlining the specifics of an ad campaign. It acts as a contract, specifying details such as the types of ads to be displayed, the duration of the campaign, targeted audiences, ad placement locations, budget, pricing models (e.g., CPM, CPC), and any special conditions or requirements. An IO ensures that both parties agree on the campaign's scope and delivery metrics, making it a crucial document for campaign management and execution.