PCBs, or polychlorinated biphenyls are a class of organic chemicals consisting of two joined benzene rings (hence "biphenyl") and any number of their ten hydrogen atoms replaced with chlorine (hence "polychlorinated"). They have have had many industrial uses, notably in electronics where they can act as fluids in transformers or capacitors.
The toxic properties of PCB compounds were noted as early as the 1930s but didn't come to prominence until the 1970s when dead seabirds were found with hight concentrations of PCBs in them. Effects of exposure to humans are well know, and include skin irritation and liver damage. To the delight of the Daily Mail there is a suspected link to certain types of cancer. Generally, they aren't soluble in water, are highly volatile and quite flammible. Some types of PCBs have similar toxic properties to dioxins. Basically, this is nasty shit that you don't want to be pouring into rivers and lakes... oh, wait.
 Contamination events
Prior to their ban in the United States in 1979, PCBs were widely flushed into rivers and waters without much thought. General Electric is believed to have flushed nearly 600 metric tons of the stuff into the Hudson river in the thirty years leading up to the ban - this also lead to a fishing ban on the Hudson as health and environmental effects became known.
 In popular culture
Zodiac by Neal Stephenson goes into great detail about the accursed stuff.