Most people think that plants need soil for them to grow but actually this is not the case. It is the nutrients contained in the soil that plants actually need.
Plants in a hydroponic system use gravel or clay balls and use either a passive or active system.
When a plant is grown in soil most of its energy is directed to the roots searching for water and nutrients in the surrounding soil. Hydroponic systems send the required nutrients to the roots directly, therefore allowing the plants energy to be spent on flourishing leaves and flowers.
Generally speaking a plants root system grown using hydroponic methods will be much smaller due to the fact that they are not searching for nutrients, this allows you to plant closer together allowing for larger yield in a smaller area.