Mediation is a term that specify the sales process
waterfall is the traditional mediation. It uses historical data to prioritize networks and call them one at a time. ie Each ad network has the opportunity to fill an ad request; however, if one ad network does not fill the request, the next ad network in the sequence gains an opportunity.
The ad server may implement a priority system. See line item priority
header bidding calls all participating networks simultaneously, enabling them to compete equally in a single, unified auction. For any given impression, the winner is always the highest paying advertiser because all networks get the same priority (including Google).