Indo Hot Blog - Basically the Earth’s atmosphere (our air) is made up oxygen, nitrogen and a dozen other gases. All of these serve to capture a certain amount of heat and keep the Earth warm. Sunlight hits the Earth and is absorbed as heat. That heat radiates outward back into the air. The molecules in the air absorb and release that heat. Certain molecules that are heavier than others radiate more heat back to earth. Without these molecules from special gases, the Earth would be frozen over at around 14 degrees below zero. 
This absorption and reflection of heat is known as the greenhouse effect. For just as in a greenhouse heat is trapped and built up by our atmosphere. So then, if all of this is supposed to happen, what is the problem?
The problem is that we keep adding to the special mixture of our air too much of certain ingredients such as Carbon dioxide. Carbon dioxide or CO2 is essential to nature. Plants use CO2 and sunlight to create sugars and fresh oxygen. Still you might think… fine so the plants will grow even better if we have extra CO2. That is what many scientists thought when they first examined global warming.
But we only have so many plants, and our oceans, which also absorb CO2, are at maximum capacity. So all this extra Carbon dioxide goes up into the atmosphere where it blocks the release of heat leaving the earth.
Gases such as CO2 that absorb heat that otherwise would have expelled to outer space are called greenhouse gases. There are quite a few of them.
Source : Globalwarming.com
