Like a nuclear bomb, a chemical or biological weapon is a weapon of mass destruction. An effective attack using a chemical or biological agent can easily kill thousands of people.
A chemical weapon is any weapon that uses a manufactured chemical to kill people. The first chemical weapon used effectively in battle was chlorine gas, which burns and destroys lung tissue. Chlorine is not an exotic chemical. Most municipal water systems use it today to kill bacteria. It is easy to manufacture from common table salt. In World War I, the German army released tons of the gas to create a cloud that the wind carried toward the enemy.
Modern chemical weapons tend to focus on agents with much greater killing power, meaning that it takes a lot less of the chemical to kill the same number of people. Many of them use the sorts of chemicals found in insecticides. When you spray your lawn or garden with a chemical to control aphids, you are, in essence, waging a chemical war on aphids.
Many of us tend to imagine a chemical weapon as a bomb or missile that releases highly toxic chemicals over a city. (For example, the movie "The Rock" featured a scenario in which terrorists tried to launch a missile loaded with the chemical VX, a nerve toxin.) But in 1995, the group Aum Shinrikyo released sarin gas, a nerve gas, in the Tokyo subway. Thousands were wounded and 12 people were killed. No giant bombs or missiles were involved -- the terrorists used small exploding canisters to release the gas in the subway.
Although the attack wasn't intentional, Blackwater Worldwide, contracted by the U.S. to protect officials in Iraq, accidentally dropped canisters of CS, a gas similar to tear gas, on soldiers and civilians at a checkpoint to clear the road for a convoy in 2005 [source: Risen].
A biological weapon uses a bacteria or virus, or in some cases toxins that come directly from bacteria, to kill people. If you were to dump a load of manure or human waste into a town's well, that would be a simple form of biological warfare -- human and animal manure contain bacteria that are deadly in a variety of ways. In the 19th century, American Indians were infected with smallpox through donated blankets.
A modern biological weapon would use a strain of bacteria or a virus that would kill thousands of people. Tom Clancy has explored the idea of biological terrorism in two books: "Executive Orders" and "Rainbow Six." In both books, the source of infection is the Ebola virus. In these plot lines, the infection is spread through small aerosol cans (like those used by insecticide products to create "bug bombs") released at conventions, or through misting systems used to cool sports venues.