A phenol-based resin is added as a binder. Rockwool also tends to increase the pH of the water. Pea gravel - this media is just simple gravel but has been graded for size and shape. It is not a porous media so it does not wick water from below and must be used in a system that provides aeration for the water. It can be used to grow bacteria as well as plants.
Different media provide different positive and negative affects for a hydroponic system.
Also, some are more available or cheaper than others are are. Some have an environmental impact greater than others do, in the ways they are gathered or manufactured. If you can find perlite, it is probably the best media you can use for a home system. Perlite is forgiving for the beginner because it will retain water for three or four days if you forget to water your system. If Perlite is not available, gravel is a good second choice because it will allow for a once a day watering and does not break down.