Answer:
Fossil fuel (coal, oil and natural gas) contains carbon that has been kept out of the earth's carbon cycle for millions of years. The earth has a natural greenhouse effect which has kept the temperature comfortable for man during that time.
We are now (since the Industrial Revolution 200 years ago) burning more and more fossil fuel (in industry, transport and to generate electricity). This extra carbon dioxide (CO2) is adding to the CO2 normally in the atmosphere and causing an enhanced, or accelerated greenhouse effect.
The greenhouse effect happens because greenhouse gases (any gas with at least 3 atoms) stop some of the sun's infrared radiation escaping out to space. The gases use the captured energy to produce heat. More and more of the greenhouse gases means more and more heat, all over the planet. Global warming of the planet.