Custom builders build a boat to customers specs. They are generally not building boats, except in the basic stages, until someone has committed to buy it. They do have a demo boat or 2 they build and sell eventually. Many are factory direct sales or they have a small dealer network.
Production Builders build boats pretty much all the time in more of an assembly line fashion. They sell them mainly through a dealer network and require dealers to order so much $$$ worth of boats per year. You can still custom order a lot of options and have the boat completed to your specs, but generally most people buy whats in stock.