The Greenhouse effect is a natural process – a kind of thermostatic process, a natural temperature control system that enables the Earth to sustain average surface temperatures in the region of 15C [15 degree celisius]. This is what sustains life – makes the Earth inhabitable.

The earth’s surface gets warm due to the visible and near-visible component of solar radiation, which passes through the transparent medium – the earth’s atmosphere. This radiation consists of shorter wavelengths of light, visible light, which is to a large extent absorbed by land, vegetation, the sea and oceans, and other water bodies– as much as two thirds of the radiation that reaches land and sea is absorbed. Ice sheets (the Arctic and Antarctic polar caps) reflect most of this radiation back. The atmosphere absorbs the least of this solar radiation. But it absorbs the shorter wavelength (ultra violet) and longer wavelength (infra red) components of solar radiation. The absorption capacity of different types of surfaces is termed as albedo.

As the Earth’s surface becomes warm, it also emits infrared radiation, the longer wavelengths of invisible radiation. Some of this radiation is trapped in the atmosphere, and results in its warming. This is called the Greenhouse Effect.

It is similar to the effect of greenhouses that enable us to stabilize temperatures artificially, to grow plants that would not survive in local climatic conditions. But the processes are different, and therefore the analogy is actually incorrect! The Greenhouses in which plants are grown are heated directly by solar radiation – the sun’s heat. Whereas the Greenhouse Effect we are referring to, which heats the Earth’s atmosphere, is a result of the infrared radiation emitted by the heated earth’s surface.

The naturally occurring gases that trap infrared radiation include water vapour, carbon dioxide, ozone, methane and nitrous oxide. These are called Greenhouse Gases (GHGs).

The Earth has the right thickness of atmosphere, and the right mix of these gases to heat the atmosphere to pleasant levels. This makes life as we know it possible on Earth. Else the Earth would heat up by day, and cool down by night.

Venus is too hot, Mars too cold, the Earth just right – until recently.
As the concentration levels of GHGs in the atmosphere change, the lower atmospheric temperature also changes. This is a natural cycle of warming and cooling over long periods of time and is a natural process of Climate Change.

The above is a compilation from the following sources:
Diagrams: What is The Greenhouse Effect?
http://www.global-greenhouse-warming.com/what-is-the-greenhouse-effect.html
This website provides basic reading material on Greenhouse effect with a simple diagram.

http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
A quick animated explanation of Global warming and the Greenhouse effect

http://www.physicalgeography.net/fundamentals/7h.html
This website is an eBook on physical geography and the chapter on Atmosphere has a detailed write-up on the Greenhouse Effect, with a lot of links to technical terms.

What is Greenhouse effect
by Jean-Marc Jancovici, September 2003
This Website answers some elementary questions on Climate Change by the author Jean-Marc Jancovici