Chemical agents, in the context of terrorism, are combinations of chemicals that yield a toxic (i.e., poisonous) effect upon exposure. That is, they are designed to kill, sicken, or harm people when they are released. There are, of course, a multitude of chemicals; only a relative few are used as chemical agents in weapons of terrorism. Chemical agents produce their effects as a result of chemistry rather than through the physical effects (e.g., blast, fragments, projectiles, heat) of conventional weapons - although when dispersed by means of an explosive device, both kinds of effects may occur. Chemical agents differ from biological agents in that they do not have a "live" biological component. Therefore, the spread of disease is not an issue with chemical agents. However, inhalation of or contact with volatile chemicals can present a major danger of mass casualties. The main differences between industrial chemical accidents and chemical terrorist incidents may be intent and magnitude. This document goes over the types of chemical agents, their effects/treatment, and potential targets.
Federal Emergency Management Agency (FEMA): http://www.fema.gov
Tool Kit for Managing the Emergency Consequences of Terrorist Incidents: Interim Planning Guide for State and Local Governments, Appendix B