Can be of a number of different types such as damaging agents like blister gases (e.g.mustard gas), blood agents like hydrogen cyanide gas, or nerve agents that work by buggering up the nervous system. These tend to be the most poisonous and have been used in terrorist attacks such as the Tokyo Subway Sarin attack.
Generally speaking, unpleasant stuff that is well worth having your S10 handy for.
Currently being used as a great way to frighten civi's and allow the MET to kick in anyones door (particulary in the Forrestgate area) that an educationally sub-normal jailbird has fingered cause he thinks it will get him off.
see also GAS GAS GAS