Volatile Organic Compounds (VOCs) are organic chemicals that have vapor pressure points at an ordinary room temperature. These high pressure points cause large numbers of molecules to evaporate from a liquid or solid form to a gas and flow into the air. Gas particles are harmful to humans and can cause health problems such as headaches, nausea, and even liver damage. Example VOCs are second hand smoke, exhaust from cars and paint.
More info on VOCs can be found in this article.