The 5 major greenhouse gases are carbon dioxide, methane, nitrous oxide, CFC-12, and CFC-11. 

Greenhouse Gases
Greenhouse gases in the atmosphere are similar to the glass covering a greenhouse. That is, they allow short
wavelength solar radiation to pass through the atmosphere to reach the earth's surface, but absorb the longer wavelength heat that is radiated back into the atmosphere from the earth. The concentration of greenhouse gases in the atmosphere thus has an effect on the average temperature at the surface of the earth. If the atmospheric concentration of greenhouse gases decreases over time, then more heat will escape through the atmosphere, and the average temperature at the earth's surface will go down. If the greenhouse gas concentration in the atmosphere increases, however, less heat will escape to outer space and the average temperature at the earth's surface will increase.

• Carbon Dioxide
• Sources of Carbon dioxide (CO₂)
  

  • Combustion of any organic fuel (wood, natural gas, fuel oil, coal, etc.)

  •  decomposition and decay of dead plant and animal matter. 

  
  

  

  Some of the carbon dioxide from the atmosphere is taken up by plants through photosynthesis; some is absorbed by the ocean waters, and some is absorbed in soil. Currently however, we're putting carbon dioxide into the atmosphere faster than it is coming out. Based on measurements that were initiated in the latter half of the 20th century, carbon dioxide levels in the atmosphere have increased from about 270 ppm before the beginning of the industrial revolution in the mid-1700s to about 380 ppm today. As shown, carbon dioxide in the atmosphere is responsible for 55% of the greenhouse effect.

• Methane

  Methane (CH₄) is the main component of natural gas, which is widely used as a fuel. The methane that we're concerned about in the atmosphere, however, doesn't come from burning natural gas (which puts carbon dioxide into the atmosphere, but not methane). The primary source of methane in the atmosphere is anaerobic decomposition of organic matter, for which methane is one of the end products. Some of the anaerobic processes (meaning in the absence of oxygen) that produce methane are in sanitary landfills, from concentrated waste organic matter (like livestock manure), from bogs and swamps, and  from termites. Methane is produced in anaerobic digesters in wastewater treatment plants, but in most cases it is either captured for use or burned in a flare.
   Methane is approximately 25 times as effective as carbon dioxide in retaining heat in the atmosphere.

• Nitrous Oxide

  Nitrous Oxide (N₂O) is probably most widely known through its use as laughing gas, a mild anesthetic. The sources for its presence in the atmosphere are a variety of agricultural and industrial sources. Nitrous oxide is part of the natural nitrogen cycle as well; it is an intermediate in the denitrification of nitrate to nitrogen gas. A significant characteristic of nitrous oxide as a greenhouse gas is its longevity, with an average persistence in the atmosphere of 120 years. Nitrous oxide is approximately 300 times as effective as carbon dioxide in retaining heat in the atmosphere.

• CFC-12 and CFC-11

  CFC-11 (CCl₃F) and CFC-12 (CCl₂F₂) are the two chlorofluorocarbons (CFCs) that have the greatest greenhouse effect. CFCs do not occur naturally. They were first created in 1928, and saw significant use as cleaning solvents, aerosol propellants, and refrigerants. Based on the effect of CFCs in destroying stratospheric ozone, a very successful global effort has essentially halted their production, so that atmospheric levels are now remaining constant or decreasing. Atmospheric concentrations increased from zero before 1928 to current levels of about 240 ppt for CFC-11 and about 530 ppt for CFC-12. At parts per trillion, the concentrations of these two CFCs are less than atmospheric carbon dioxide concentrations by a factor of 10⁶. They still have some greenhouse gas effect, because they are 5,000 to 10,000 times as effective as carbon dioxide in retaining heat in the atmosphere.
  
  Source :  BrightHub .com