Organic chemistry is the study of carbon based compounds. The reason why there's a specific branch dedicated for this is because there are more carbon based compounds than all the other compounds combined. While carbon being the main element, there are few other elements (for instance hydrogen, oxygen, nitrogen, sulfur, silicon, phosphorous, and halogens) that you normally find in carbon based compounds. Organic chemistry focuses on structures, properties, composition, reactions, and synthesize of carbon based compounds. Organic chemistry has many applications. Specially in pharmaceuticals, food, petrochemicals, cosmetics, paints, explosives etc.