The term "weapons of mass destruction", used to encompass nuclear, biological, and chemical weapons, is misleading, politically dangerous, and cannot be justified on grounds of military efficiency.
-Gert G. Harigel of the Carnegie Endowment for International Peace
I am unaware of an official list of items that fall under the heading, "weapons of mass destruction", but the 2 of the 3 items most people regard as weapons of mass destruction are, in fact, not massively destructive, either in terms of human life or property damage. I commend fellow noders for writing much on the subjects, so I will only stick to the points that relate to how they're not any more massively destructive than bombs or bullets.
The least destructive of the three is easily chemical weapons. To most people chemical weapons are poisonous liquids or vapors, even though conventional munitions are made possible through chemistry. When referring to chemical weapons in this node, I mean poisons. The most effective use of chemical weapons was that of the Nazi death camps, primarily against Jewish peoples. These killings were executed in a very controlled, indoor, environment. A variety of limiting factors come into play when chemical weapons are used in the field.
Meteorological factors come into play, obviously. Aptly named NBCthreat pointed out in the chemical weapons node that chemical weapons are not gases, but in fact, vapors. A vapor is defined as, "diffused matter suspended floating in the air". The important distinction with gas is that vapor tends to settle which, in turn, makes it extremely difficult to disperse while retaining enough of a concentration to affect possible victims. Wind speed and direction play a very important role in classical warfare, as chemical vapors deployed over enemy troops could easily be blown toward friendly troops. In a situation where everyone is an enemy, a major city for example, the optimal weather conditions would be that of a slight steady wind, enough to spread an agent across an area, but not so much as to disperse it to the point where it is no longer effective.
Chemical weapons can also be easily protected against, at some inconvience to troops and civilians. Gas masks and protective suits can be worn to protect against airborne agents by troops, and a proclamation to civilians to close windows and stay indoors for a few hours would save countless lives. Biological weapons are much harder to protect against, and nuclear weapons are virtually impossible.
Biological weapons are much more deadly than chemical weapons, due to their rapid manifestation within a populace and staying power, in the case of contagious agents. Other agents, such as anthrax, are not communicable from person to person, and behave much like chemical weapons. The dispersion of Anthrax falls ill of many of the same meteorological factors, which are detailed in Jeremy F's anthrax writeup.
The deadliest forms of biological weapons are those contagious from person to person diseases from nature that were deadly, but are now deadlier thanks to man. Many disease samples were collected and weaponized by both the United States and Soviet Union in the not-so-great-idea that was the arms race. Diseases were weaponized with the goal of making them very hard for the enemy to contain. Naturally contagious and deadly diseases such as smallpox were chosen to be weaponized in order to increase their effectiveness, most often by designing strains resistant to treatment. While powerful countries no doubt can manufacture strains of diseases with malicious intent, it is debated whether a terrorist organization has the resources to manufacture a disease, it is very possible that they could steal pathogens from neglected unguarded former Soviet Union facilities. Distribution of a contagious would be relatively easy, as a suicide soldier could easy infect himself with the disease, and hop around the target country, making detection and quarantine virtually impossible because of seemingly random outbreaks.
Biological weapons, although much harder to safeguard against than chemical weapons, can be prevented the same as any disease. Hand washing and basic hygiene will safeguard you against thousands of diseases, manmade or not. Vaccines are also available to certain parts of society, currently soldiers and those within the healthcare infrastructure are allegeable for smallpox vaccines. Biological warfare is very dangerous, although it hasn't been seen on a massive scale in modern times.
The grouping of biological and chemical weapons with nuclear weapons is a dangerous misconception which breeds fear. While biological weapons could someday reach proportions similar to nuclear weapons, an incident of that scale hasn't happened in modern times. "Weapons of Mass Destruction" is a term used to instill fear, and is often leveraged for political advantage when in fact the threat of WMDs is insignificant (for now)