Afternic describes this as follows (not complete, and with some outdated numbers, but the general info is still correct)
The Premium Network differs from the Standard Network two important ways. First, the resellers promote the domain name with their buy now price for immediate purchase, without counteroffers from the seller or bidding at auction. Second, fast-transfer domains are transferred to the buyer immediately.
Making your domain a fast-transfer domain gives you the highest level of exposure--75 million searches by prospective buyers each month at the highest average sales prices. Fast-transfer domains must have a buy now price and be registered through GoDaddy, DynaDot, DomainDiscover, TierraNet, Network Solutions, Register.com, Register.com Global Partnership Group, eNom, eNomCentral, BulkRegister, Name.com, Hover, EuroDNS, DomainMonster.com, 123 Reg, DomainBox, Heart Internet, Regtons, MyDomain, Domain.com, Dotster, Namesilo, Hexonet, NamePal, Epik, Rebel.com, or Internet.bs.
The most recent overview of all Afternic Premium Network partners can be found on this page: