There are a handful of greenhouse gases; the most talked about these days is carbon dioxide (CO2.) The others are methane, nitrous oxide, ozone, CFCs, and believe it or not, water vapor. These greenhouse gases adsorb, reflect, and emit infrared radiation. Without greenhouse gases, heat in the form of infrared radiation would escape the Earth's atmosphere instead of being reflected back to the surface and we would be wear parkas in July.
Greenhouse gases are unique among other atmospheric gases in their ability to interact with infrared radiation. Unlike diatomic nitrogen (N2) and oxygen (O2), greenhouse gases have a net charge in their dipole moment. Basically, this means that they can react with infrared radiation.