Hydroponics uses many different mediums to plant instead of dirt; dirt is messy and not very conducive to good hydroponics because dirt has many impurities and diseases. Hydroponics uses mediums like coco coir, Leca, vermiculite, perlite and many others, each has advantages and disadvantages.

I personally prefer DWC (Deep Water Culture) especially inside the home because it's less messy,

However if you are doing it for business I recommend  Leca rocks, unlike the others it is easier to work with and can be reused if properly cleaned. Each of the mediums has different uses although I find that Leca rocks are the most versatile for planting in a greenhouse hydroponics system. Leca is porous and can retain a good amount of water. 

One advantage of Leca and other mediums is the ability to retain water and if for example you have a power outage you can always water the plants manually but with a DWC this is not possible because you need Air flowing through the roots, if power is not restored within two days your plants will die. The picture to the left is the root ball of a tomato plant, notice the roots weaved through the Leca and this is what makes mediums difficult to work with. 

Mediums are messy, expensive and work intensive when cleaning your system and it's why I use only DWC inside the home. Leca can be cleaned if left out in the sun to dry for a couple of weeks washed and reused over many times other mediums like coco, perlite and vermiculite are difficult to wash and are best discarded and fresh medium added.