Greenhouse gases are gaseous elements of the atmosphere that absorb and emit radiation.  They exist naturally in the Earth's atmosphere and are part of what keeps the Earth warm and habitable.  The Earth is balanced like a greenhouse whereby the atmosphere allows heat from the sun in, but only lets a certain amount out.  Thus, the Earth can support life just like a greenhouse does. Examples of greenhouse gases include carbon dioxide, methane, and nitrous oxide.
 
Greenhouse gases are most commonly discussed with respect to global warming.While the right amount of greenhouse gases in the atmosphere keeps the Earth at a temperature ideal for sustaining life, too much of them makes the Earth much hotter than it should be. A portion of greenhouse gases occur naturally; the rest are human-made. Because of human-caused greenhouse gases, the Earth is heating up, leading to such drastic effects as global warming and climate change.