"A group of natural and man-made compounds that, when present in the atmosphere, allow radiant energy (i.e., light) from the sun to pass through to warm the earth's surface but tend to block infrared energy (i.e., heat) reflected back from the earth, thus trapping the energy in the atmosphere in what is referred to as the greenhouse effect. Water vapor, carbon dioxide, methane, and nitrous oxide are greenhouse gases produced by both natural and human processes. Entirely man-made greenhouse gases include hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride."
source: http://www.mondaq.com/unitedstates/article.asp?articleid=87596
"Greenhouse gases are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The main greenhouse gas in the earth's atmosphere are water vapour, carbon dioxide, methane, nitrous oxide and ozone. In our solar system, the atmopshere of Venus, Mars and Titan also contain gases that cause greenhouse effects. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would be on average about 33 °C (59 °F) colder than at present."
