Global warming is the word given to the phenomenon where certain gases, chiefly CO2 and CH4 trap heat in the atmosphere of the earth. This causes a small increase in temperature, which has far-reaching consequences for weather patterns.
What happens is that the warming effect takes place disproportionately at the poles. The reduced temperature differential between the poles and the equator changes the dynamic of global climate.
Global warming is a misleading term, because the actual warming effect is only a matter of one or two degrees, hardly noticeable to most people. The real problem is that this changes wind patterns and causes climates to change, which is far more serious. Climate change is probably a more appropriate term for what is happening therefore. The effects of climate change on food and water supplies in the world could be very serious.