Hydroponics is the processing of growing plants in water without soil. The plants gain their nutritional requirements from mineral nutrients dissolved in the water. This system was developed after it was discovered in the 18th century that plants absorb nutrients from soil as ionic solutions (ie. dissolved in water).
Some terrestrial plants require an inert medium (eg. gravel) to be grown hydroponically.
Hydroponics are typically used for crops where advantages include: no soil, water can be re-used, nutrients can be completely controlled, no nutrient pollution, stable/high yields, easy to harvest, easier to control pests. The primary disadvantage is that there is no soil to act as a chemical buffer, so any failure in the hydroponic solution will quickly lead to plant death.
Use this tag for all questions about growing plants using hydroponics.