Dirty bomb is the colloquial name given to a Radiological Dispersal Device, a bomb that combines a conventional explosive with radioactive material. The conventional explosive embeds radioactive material within it, so that when it is detonated it disperses the radioactive agent. The U.S. Nuclear Regulatory Commission has indicated that most dirty bombs would not contain enough radiation to cause significant lethal damage. Thus, although they use radioactive materials, dirty bombs are "in no way similar to a nuclear weapon." Their primary use is to terrorize, rather than cause widespread damage.
Another way of dispersing radioactive materials is to simply leave them somewhere where they will release into the air, "creating fear and possibly panic, and requiring potentially costly clean-up."
For more terrorist weapons, see:Glossary of Terrorist Weapons