VOC stands for Volatile Organic Compounds. VOCs are chemical compounds that are carbon based and are in the form of vapor when at room temperature. In this case, organic is not a good thing, and indicates that these compounds do not occur naturally, but only as by-products of man-made substances.
Products and materials which contain volatile organic compounds release them as vapors through a process known as outgassing, which is lot like evaporation. The compounds are referred to as volatile because they are made of unstable molecules which are easily released into the atmosphere, particularly in environments of higher heat and humidity.