Simply put, hydroponics is the growing of plants without soil. The word "hydroponics" comes from the Greek word hydro, which means "water" and ponos, which means "labor or water-working."

Typical Dirt Gardening:

All plant leaves need light, oxygen and carbon dioxide. Plant root systems require water, nutrients and oxygen. When plants are grown normally (in soil) water takes nutrients from the soil and carries them to the plant roots. The water and nutrients are taken up by the roots to feed plant growth. Soil drainage then allows water to be replaced by air in the gaps between soil grains. This supplies the roots with oxygen.

Hydroponic Gardening:

In hydroponic plant systems, you dissolve the nutrients in water. Soil is replaced with a "growing medium" - a soil substitute - that holds the roots and supplies them with water, nutrients and oxygen. You can deliver the nutrient solution a couple of ways: You can drip feed it to each plant, or you can flood the root chamber, then drain it out. These methods require a pump and timer to circulate the nutrients through the roots. You can also grow the plant roots in the air by spraying them with a fine mist of nutrient solution, or grow them by aerating the solution under each root mass with an air pump.

Actually, six basic types of hydroponic systems make up the basis of all hydroponic gardening.

Wick
Water Culture
Ebb and Flow (or Flood