Industrial grade PCBs (CAS #27323-18-8) are mixtures of various congeners and isomers of chlorinated biphenyl; the most common commercial PCBs contain an average of from 1 to 3 chlorine atoms per molecule.
They are no longer (since 1977) manufactured in the US, and their use and disposal is totally regulated by the EPA (incineration at temperatures >1100[sup]o[/sup]C, long residence times, specially designed incinerators).
Former uses were wide-ranging and based on PCBs unique combination of properties: fire resistance, thermal and oxidative stability, electrical characteristics (high dielectric constants, high volume resistivities and dielectric strength), solvency in most organic solvents (but not in water, glycerol and glycols), inertness, and liquid range.
The largest use was in electrical equipment such as transformers and capacitors.
If PCBs properties are of interest I assume Monsanto could be a good source of information.