Hydroponics is all about "ways of growing plants without soil"
When you take soil away from a plant you are removing natural mechanisms that the plant has come to rely upon. By removing what nature provides, you have a simple advantage - you can control the forces of nature, providing the positive forces, and eliminating the negative forces (eg. poor environmental conditions or exposure to disease). Doing this is not simple though. It requires a knowledge of plant and chemical science as well as horticultural technique. You can control a plant better, but only if you have the knowledge to control it properly.
The 4th International Congress of Soilless Culture defined the different systems of hydroponics as follows:
Water Culture: The roots of plants are submerged in a nutrient solution (e.g. Nutrient Film Technique or NFT).
Sand Culture: The roots of plants are placed in a solid aggregate composed of particles with a diameter of less than 3mm (e.g. sand, perlite, plastics, and other in organic materials).
Gravel Culture: The roots of plants are grown in a solid aggregate composed of particles greater than 3 mm diameter (e.g. gravel, basalt, scoria, pumice, plastic, and other inorganic materials).
Vermiculaponics: The rots of plants are grown in vermiculite or a mixture containing vermiculite.
Rockwool Culture: The roots of plants are grown in Rockwool, Glasswool, or some similar material.
Hydroculture: This can refer to all forms of hydroponics but is more commonly used to refer to growing ornamental or decorative plants inside.
Aquaponics is another technique that has become popular, but it is even more complex, because it links a hydroponic system to a fish farming system with water flowing from one into the other then back to the first. It may be best to not attempt aquaponics until you have first mastered hydroponics.
What Is Involved?
Soil does four main things for the plants which grow in it. It provides:
Support - stops the plant falling over or blowing away by providing anchorage for the plant's roots.
Water - via the plant's roots.
Air - through the roots (yes, the plant absorbs gas out of the air through its roots! A plant's roots can starve for air just as much as they can starve for water!).
Nutrients - food in the form of very simple types of chemicals.
To be successful, hydroponics needs to adequately cater for these FOUR FUNCTIONS, which are usually handled by the soil.
If the roots are not grown in a hydroponic media which will enable adequate support for the plant, then a trellis or some other artificial means must be provided to support the plant.
It is essential to maintain a balance between the moisture and air content of the root environment. With some plants the air which is dissolved in water might be adequate but for the majority of plants, total immersion in water will result in air starvation and death. In anything other than water culture (aggregate culture), the ability of the medium to hold moisture and the relationship between this characteristic and their air-holding ability is critical.
A medium which drains very easily usually holds water well but might dry out very easily. This type of medium may need watering very frequently. A medium which holds water very well could become waterlogged and the plants could suffer from a lack of air if they are watered too often.