In waterfall mediation, each network is called in an order determined by the mediator. Usually this order is determined by which ad network the mediation algorithm predicts will return the best paying ad. But sometimes the networks are placed in a fixed order or a particular network is guaranteed "first call" in the mediation waterfall.
In programmatic mediation, each time there is an opportunity to show an ad, all available networks bid in real-time for the opportunity to show an ad, and the highest bidding ad is shown. Programmatic mediation goes by many names in the industry: header bidding, FairBid, advanced bidding, parallel bidding, and other terms are used.