Hydroponics can easily be called a greatest innovation in the field of indoor cannabis cultivation. Although it was originally designed for agriculture, there is some scientific evidence that the whole industry developed mainly thanks to marijuana connoisseurs around the world.
They basic idea behind hydro is the removal of soil. Instead of watering the pots filled with soil, which in turns transfers the moisture and nutrition to the plant, hydroponically cultivated plants get their feed delivered directly to the root system.
Though operating a hydro set is somewhat more complicated than the most basic soil based cultivation, one can grasp it pretty quickly. Some beginners kits are also available.
The reason for abandoning soil is that it is at the bottom of most problems - starting from pests, through mould to overflooding. Hydroponics allows you to cut down on the nutrients use. Since they are delivered directly to the roots - intravenously, so to speak - there are virtually no loss (as is the case with soil, where some of the nutrients never really reach the plant).
In absence of soil, some sort of support for the plants root system is needed - most often rockwool, coco or liaflor (or some mix of those) is being used. The key to a good hydroponics substrate is that water flows through it unobstructed.