A greenhouse gas (often abbreviated as GHG) is a gas that both absorbs and emits radiation in the infrared range, commonly called thermal radiation or heat. When present in the atmosphere, these gases trap radiation in the form of heat, causing a warming process called the greenhouse effect. The presence of four major greenhouse gases, namely water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the Earth's atmosphere keeps the average temperature of 15º C (59º F), whereas without the greenhouse effect the average temperature would be a frosty -18º C (0º F).
Some concentration of greenhouse gases in the atmosphere is normal, and in fact necessary for life on Earth as we know it. It is normal for the concentrations of these gases to fluctuate over time, causing the average global temperature to vary over a period of hundreds of thousands or millions of years. These natural fluctuations are slow, especially in comparison to the atmospheric lifetime of GHGs, or the time it takes for a gas emitted to the atmosphere to be removed by the ocean, the biosphere (the living systems of Earth), or by other means. Because the changes in GHG concentrations are slow and buffered, life on Earth has time to adapt to the corresponding changes in temperature, for the most part avoiding mass extinction of other catastrophic events.
With the dawn of the human industrial age, however, came vast additions of greenhouse gases to the atmosphere, mostly from the combustion of fossil fuels (for example gasoline, coal, and oil). These emissions, or infusions of thermally active gases into the atmosphere came at a much higher rate than anything previously seen on Earth. Whereas with the slow increases seen before, the anthropogenic emissions cause a rapid increase in the atmospheric concentrations of GHGs, as you can see in this 2000-year record from the IPCC 2007 Annual Report.
Water vapor is not shown on this plot, not because it is not influenced by anthropogenic changes, but because it is highly variable for other reasons. The three gases shown, in addition to tropospheric ozone (O3 in the lower atmosphere) and a class of compounds called halocarbons (organic compounds containing chlorine, bromine, or fluorine, for example freon) are the major greenhouse gases emitted by human activities. Their radiative forcings, or the amount to which each gas can change the atmospheric temperature, are compared below in another plot from the IPCC 2007 Annual Report.
From this plot it is obvious that the major anthropogenic contributor to climate change is carbon dioxide, or CO2. This is because CO2 is emitted in amounts that far exceed those of any other greenhouse gas. The massive increases in atmospheric GHG concentrations since the industrial era began are shown below in a figure from the IPCC 2007 report. Notice that the rate of temperature increase (the steepness of the slope) has become larger as more of the world becomes industrialized.
Gas that both absorbs and emits radiation in the infrared range
The major human created contributor to climate change is carbon dioxide