A class of organic chemicals, described as a planar molecule bonded with halogens (e.g. chlorine or bromine). Dioxins have been used as herbicide (Agent Orange), before concerns were raised over long-term toxicity and transmission through the food chain. Now it is considered a toxic industrial waste.
Dioxin accumulates in the body over time; it does not seem to be eliminated by the natural filtering processes of the kidneys, spleen, etc. It is a known carcinogen (i.e. promotes cancer). At sufficient exposure levels, there is a "switch" effect and the victim starts to die. It causes abnormal cell growth and differentiation. Common signs of sub-lethal dioxin exposure in developmental stages are severe cystic acne and low testicular mass. It can provoke immune hypersensitivity, or depress the immune system. General sensitivity in mammals is widely varied; in some ways, dioxins behave more like hormones than simple toxins.

Dioxin compounds are common covalent chlorine pollutants. To use Neal Stephenson's metaphor from Zodiac, a dioxin molecule is basically a six-pack (or more, or less) of covalent chlorine atoms -- or a raft of covalent chlorine gunmen come to shoot up your chromosomes. Covalent chlorine is quite ridiculously bad for you -- and for pretty much any other organism.

Dioxins are produced by several industrial processes, including the manufacture of paper from wood pulp. (Hemp paper does not involve dioxins.) Several plastics and other petrochemicals also produce dioxins when burned incompletely.

A dioxin compound was the contaminant in Agent Orange that caused a lot of folks in Vietnam to get sick from it.

Dioxins are related to polychlorinated biphenyls, which differ in that they have phenyl rather than benzene rings.

Dioxin can damage the male reproductive system. After the city of Seveso in Italy had an explosion that released dioxin, there was a reduction in the number of males born.

Research done by Paolo Mocarelli has shown that boys who were younger than 19 at the time of the accident went on to have many fewer sons than normal - the usual rate is 51 sons per 100 babies, but in their case, it was only 38 sons per 100. Males older than around 19 were not affected, possibly because they had developed already.

from http://data.intelihealth.com/pcn/menshealth/00283744.htm

   2        3       Dioxins contain one to eight
    \      /        chlorine atoms covalently 
     C----C         bonded at any of the numbered
    /  __  \        locations in the model to the
1--C  (  )  C--4    left.  Closely related to
    \      /        dioxins are furans, in which
     C----C         one ether bond (C-O-C) is 
    /      \        replaced by a direct C-C bond.
   O        O       Also related are PCBs, which 
    \      /        would be two benzene rings
     C----C         attached by a single C-C bond
    /  __  \        and chlorinated with one to 
9--C  (__)  C--6    ten chlorine atoms.  Dioxins
    \      /        with Cl atoms at the 2, 3, 7,
     C----C         and 8 positions are much more 
    /      \        toxic than less chlorinated
   8        7       species.

Studies done by Bruce N. Ames of Berkeley University suggest that exposure to low doses of certain dioxins reduces the cancer risk from subsequent exposure to other carcinogens. Ironically, the same doses when administered after carcinogen exposure increase cancer risk.