Any preparation of Cannabis grown using artificial hydroponics techniques. In other words, hydro is Cannabis which was not grown outside, nor grown inside with natural sunlight and soil. Many smokers argue that hydro is superior to natural weed because hydroponics techniques can be more controlled and turn out better product. However, picky bud smokers might prefer naturally-grown premium strains of Cannabis.
Many dealers charge more money for hydro than "regular" weed, especially when dealing with novice smokers. It is hard to say whether hydro is in fact superior to "regular" weed. Much of this depends on the grower himself, the procedure used, and the care taken.