Hydroponics is a horticultural production system that does not use soil (soilless) and plant nutrients are supplied to the plant dissolved in water - 'ready to use'.
There are three main types of hydroponic systems and a huge number of variations and products that can be used. Hydroponic systems can be managed as closed or as open systems. Recycling of nutrient solutions can dramactically increase the water and nutrient efficiency of hydroponics - which is already one of the most water and nutrient efficient horticultural production systems available!
All water in a hydroponic system should be disinfected before use and prior to recirculating for re-use. There are various ways to disinfect water, including ultrafiltration.
Hydroponics can be very technical and precise, though there is also simple hydroponics. It is important to use the appropriate hydroponic system for your situation.