VOCs: Volatile Organic Compounds
In general, volatile organic compounds (VOCs, also known as volatile organic chemicals or volatile organic constituents) are carbon-containing chemicals that tend to evaporate readily, but there is no universally-accepted technical definition. Some sources define VOCs as organic chemicals that boil at less than 200 degrees Fahrenheit; other definitions reference a vapor pressure of greater than 0.1 millimeters of mercury under standard conditions. The likely reason for the lack of a standard definition is that volatility is a continuous variable, like temperature, and not a binary one.
The US EPA's Terms of Environment (now archived) defines a VOC as "Any organic compound that participates in atmospheric photochemical reactions except those designated by EPA as having negligible photochemical reactivity." If that's not confusing enough check out the defintion in the U.S. Code of Federal Regulations at 40 CFR Part 51.100(s) (Link is via the Iowa Department of Natural Resources). Wikipedia adds to the fun by throwing in some international definitions.
However defined, there are many different VOCs. Environmental regulations focus on a relatively small fraction of those, generally VOCs that see wide industrial use, are breakdown products of industrial chemicals, and/or are compounds known to be particularly toxic.
There are many laboratory methods for analyzing VOCs. One of the most widespread is EPA's SW-846 Method 8260. However, there are several variants of this method, each of which lists a somewhat different set of chemicals, but all of the variants include most of the volatile organic chemicals that attract the lion's share of interest at hazardous waste sites. Here is a list of the chemicals included in Method 8260B:
2-Chloroethyl vinyl ether