A net cup holds the plant and the growing medium in place, while the roots reach down into the solution. The medium supports the plant, holds some moisture and lets air reach the roots.
Clay pebbles
Reusable and inert, so they do not change the pH much. They drain well and need a good rinse before reuse.
Rockwool
Holds a lot of water and air. Soak it at the right pH before use, because the raw material can affect the solution.
Coco coir
Holds water well and is a common choice for many systems. Rinse it first, since it can release salts.
Perlite
Light and airy, which helps oxygen reach the roots. It holds little water on its own, so it is often mixed with other media.