Earth's atmosphere contains certain gases called greenhouse gases, which are mostly water vapor and carbon dioxide.
These gases act to keep the lower layers of the atmosphere warmer that they otherwise would be without those gases.
“Global Warming” is the expected slow, gradual warming of the lower layers of the Earth’s lower atmosphere by the slowly increasing concentrations of man-made greenhouse gases, primarily carbon dioxide, and to a lesser extent methane.
These gases trap infrared radiation, which is the “heat radiation” that cools the Earth.
In order for the Earth to remain at a constant temperature, the Earth must lose as much energy through infrared radiation as it gains from the sun.
This concept is called energy balance.
The burning of fossil fuels, mainly petroleum and coal, produces carbon dioxide as one of the by-products.
As of 2003, the concentration of carbon dioxide is over 50% higher than it was before the start of the industrial revolution in the late 1800's, which is when the burning of fossil fuels really took off.