The greenhouse effect is the phenomenon where the Earth's atmosphere traps solar radiation that is being radiated from the Earth's surface. This is caused by the presence of greenhouse gases in the Earth's atmosphere. The primary greenhouse gases are carbon dioxide, methane, water vapor, nitrous oxide, and ozone. There are also hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride which are powerful greenhouse gases but they are being emitted in smaller quantities.
Figure 1. U.S. Anthropogenic Greenhouse Gas Emissions by Gas, 2001 (Million Metric Tons of Carbon Equivalent) 
Figure 1 shows the emissions of all the greenhouse gases. Carbon dioxide makes up about 80% of it mainly because it is the main by product of burning fossil fuels such as petroleum and natural gas. Methane is the next biggest and it also comes from energy use such as agriculture and oil and gas operations. The HFCs, PFCs are the hydrofluorocarbons and perfluorocarbons they come from propellants, refrigerants, and other industrial processes. They have smaller quantities but have a high global warming potential (GWP). GWP is the measure of how much heat a greenhouse gas can trap in the atmosphere, so even though HFCs and PFCs only make up a small portion they still have a huge impact on the greenhouse effect.
Figure 2. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide
Figure 2. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide
http://www.eia.gov/oiaf/1605/ggccebro/chapter1.htmlFigure 2 shows the the concentration and the emission of carbon dioxide over the last 150 years or so. Around 1850 when the industrial revolution began the emission started to increase rapidly, causing the overall concentration to increase. The increase is because the burning of fossil fuels went way up, because the need for energy went up. Energy consumption is the main cause for the increase in greenhouse gases. Heating and cooling, transportation, and agriculture are three big energy consumers which all produce large amounts of greenhouse gases. As the concentration of greenhouses gases increases in the atmosphere the amount of heat trapped will increase, causing the Earth's surface temperature to rise. The rise in surface temperature has many negative impacts such as the melting of the ice caps. As the ice caps melt the ocean level will rise which could lead to many people losing their homes becuase they live close to the ocean. All of these problems can be avoided if greenhouse gas emissions start to decrease. There are many ideas on how that could be done but none have been taken into effect.