Hydroponics grows plants without soil. The roots sit in water, or in a material that only holds them in place, and the nutrients are mixed into that water.
Deep water culture
Plants rest in net pots over a reservoir. The roots stay in the nutrient water. Air is added so the roots can breathe.
Ebb and flow
Pots of inert media sit in a tray. Nutrient water floods the tray, then drains back to a reservoir. The roots get water, then air, by turns. The media holds the plant. It is not soil and it does not feed the plant.
Nutrient film technique
A shallow stream of nutrient water runs along a channel, past bare roots. The stream is only a film. Most of the root mat stays moist and in the air, which is where the oxygen comes from.
Aeroponics
Roots hang in air. A mist or spray wets them. There is no soil and no bed of gravel around the roots.
The Kratky Greens Station Plan is a still reservoir: an opaque tote, no pump, and no air stone. It is the simple end of deep water culture, for greens at home, not a circulating farm.