Closed-loop cooling solutions are purchased as a single unit and require no after-market assembly or filling by the user. Good examples are the Corsair H series liquid coolers. They come prefilled and require no further maintenance than mounting.
Custom-loop cooling solutions are created from individual pieces purchased by the user and installed to the user's own design. The waterblock, radiator, pump, reservoir, tubing, fillports, and any additional components are each purchased individually. While all custom loops have a lot in common functionally, each design is unique.