WMD is an acronym that stands for "Weapons of Mass Destruction." This term is generally used to refer to any type of weapon that is much more destructive than conventional weapons such as bombs (other than nuclear bombs). There are typically said to be three types of weapons of mass destruction. These are chemical weapons (such as the poison gasses used in World War I), biological weapons (like if a country were to infect the animals of another country with something like anthrax), and nuclear weapons. These three kinds of weapons are seen as qualitatively different from conventional weapons and therefore deserving of their own separate category.
Weapons of Mass Destruction