Greenhouse Effect

Carbon Dioxide and water vapour have important roles to play in maintaining a fairly uniform temperature on the Earth's surface.
These gases allow the visible radiation from the sun to reach the Earth's surface but trap some of the infrared radiation which is emitted by the Earth. Energy is thus retained in the atmosphere.
This produces a warming effect known as the greenhouse effect, and gases which encourage or cause this process are known as greenhouse gases.

Defination:

Greenhouse gases are gaseous constituents of the atmosphere that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds.

adapted from http://en.wikipedia.org/wiki/Greenhouse_gas

Therefore, from the defination, we can also infer that water vapour, which is present in clouds, is a greenhouse gas.
Carbon Dioxide and Methane are greenhouse gases as well.

Because of industralization, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.
This causes a phenomenon known as global warming, whereby there is an excess of greenhouse gas such as carbon dioxide building up in the atmosphere at an higher rate than photosynthesis can remove. This causes an increase in the Earth's average temperature.

Sources of these greenhouse gases can be from cars and factories, where combustion of fossil fuels is needed.

Consequences of an increase in global warming:

1.Melting of large quantities of ice in the North and South Pole, which will cause levels of oceans to rise and flood low lying countries.

2.Decrease in crop yields world-wide, due to change in climate.
Areas currently covered by vegetation may become deserts.

3.Rapid Evaporation of water from the Earth's surface. When this happens, carbon dioxide dissolved in the oceabs will be driven out into the atmosphere, which adds on further to the greenhouse effect.