The Green

Global Warming: "An increase in the average temperature of the earth's atmosphere, especially a sustained increase sufficient to cause climatic change." (Wikipedia) [en.wikipedia.org]

This increase in temperature is caused by an increase in greenhouse gases in the atmosphere of the planet. These greenhouse gases do two things: they increase the amount of heat absorbed by the earth's atmosphere and they also prevent more heat from dissipating into outer space. The end result is that we have more and more heat trapped in the atmosphere, which in turn increases the temperature of the air and water found on the planet. This heat increases the energy available to climatic systems like wind and water currents. By far the most widely known effect of global warming would be the melting of the polar ice caps. This would result in water levels rising all around the world; your favorite beaches would be no more! Rivers and farmlands would also be covered in salt water, something not seen since the roman technique of salting farm lands.

Loosely speaking, you might be able to visualize global warming by imagining that sun tan lotion is providing the same function for your skin that the atmosphere provides for the air and water of the planet. Without sun tan lotion, your skin absorbs more heat radiation from the sun, thereby increasing the temperature of your skin to the point that it will burn. Likewise, trapping heat in the atmosphere of the planet heats up the air and water inside the atmosphere.