A marketplace user is the login you're registered with in the zanox marketplace. It does not yet identify you as either advertiser or publisher but simply as user. With your marketplace user login you only have access to the User Dashboard where you can manage and link new programmes to your user.
A marketplace programme identifies you as an advertiser operating a specific partner programme in the zanox affiliate network and holds all information relevant to your advertiser activities (programme settings and profile, publishers, ad media, commissions, etc.). You don't have to log in separately to your programme account but can simply select it from the User Dashboard or the account switcher at the top of the page.