Greenhouse gases (GG) trap the sunlight reflected off the earth surface within the atmosphere, under normal circumstances the amount of energy absorbed by the earth should be balanced by the amount reflected back out into space keeping the earths temperature constant. Increase GG traps the heat and contributes to global warming.

More info:
http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html
http://www.ieagreen.org.uk/
http://en.wikipedia.org/wiki/Greenhouse_gas
http://yosemite.epa.gov/OAR/globalwarming.nsf/content/Emissions.html