Paradichlorobenzene, also known as 1,4-dichlorobenzene, is an industrially manufactured chemical used principally as a disinfectant and a pesticide. Its characteristic sharp smell is familiar to anyone who has used a urinal in most parts of the world - the infamous 'urinal cake' is a puck made of this waxy substance which is intended to disinfect the urinal. It is used for this purpose both because of its disinfectant qualities and because it is nearly insoluble in water, meaning that the urinal cake sticks around for quite some time. It will vaporize (sublime) into the air directly from solid form at room temperature.
In pesticidal use, it is found in mothballs, where it has displaced other substances over time in an evolution towards more-effective and less generally toxic ingredients. It is also used as a specific pesticide in industrial agriculture, being effective against certain infestations. This is not to say it is completely safe; it is acknowledged to be a carcinogen at high concentrations, and the EPA and OSHA have maximum permissible limits for water supplies and workplaces.
Its molecular formula is C6H4Cl2. It is made, according to U.S. Patent 4777305, by reacting chlorobenzene with free chlorine in the presence of a zeolite catalyst under conditions of heat and pressure (usually produced with steam).