I've used cruisecompete a few times. Basically you provide your itinerary (ship, date) and cabin category, number of people. There are travel agents that submit their quotes to you. They take a discount on the commission they get, or even offer hotel vouchers, free or discount insurance, etc.

Many cruise lines do not allow TA's to advertise cheaper rates publicly, so you won't see any discount rates advertised by the other TA sites. But in cruisecompete they provide quotes/bids to win your business.