"Greenhouse gases" are gases that trap heat in the earth's atmosphere. This heating creates a number of effects on the climate of the planet, increasing evaporation of water over oceans and land in some areas, causing droughts in some areas while increasing rainfall in others. As well, a warmer atmosphere increases the rate of melting of glaciers and the polar ice caps, with the potential for significantly raising the ocean's water levels.
Some greenhouse gases hurt the Ozone molecules in the stratosphere layer earth's atmosphere. This beneficial layer extends from about 6 to 30 miles from the earth's surface and protects the planet's life forms from the sun's harmful ultraviolet (UV) rays. This natural shield has been gradually depleted by man-made chemicals like chlorofluorocarbons (CFCs) allowing more UV radiation to reach the ground increasing rates of skin cancer, cataracts, and other health and environmental problems. (This contrasts to harmful ground-level ozone, commonly called urban "smog", which affects crops, trees, and children with asthma)
Some greenhouse gases such as carbon dioxide occur naturally and are emitted to the atmosphere through borough natural processes and human activities. Other greenhouse gases (e.g., fluorinated gases) are created and emitted solely through human activities. The principal greenhouse gases that enter the atmosphere because of human activities are:
Carbon Dioxide (CO2): Carbon dioxide enters the atmosphere through the burning of fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products, and also as a result of other chemical reactions (e.g., manufacture of cement). Carbon dioxide is also removed from the atmosphere (or "sequestered") when it is absorbed by plants as part of the biological carbon cycle (they breathe Co2 and exhaust pure 02). More about Carbon dioxide
Methane (CH4): Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also come from livestock's digestive systems, other agricultural practices, and by the decay of organic waste in municipal waste landfills and from wastewater treatment. More about Methane
Nitrous Oxide (N2O): Nitrous oxide is emitted during agricultural activities (when synthetic nitrogen fertilizers are used in crop production) and industrial activities including the combustion of fossil fuels and solid waste. More about Nitrous oxide
Fluorinated Gases: Hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes, including electricity transmission, semi-conductor manufacture, aluminum and magnesium production & processing. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs, and halons), many of which have already been banned by UN convention. These gases are typically emitted in relatively smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases ("High GWP gases"). More about Flourinated Gases
For those interested, the EPA (US's Environmental Protection Agency) has a Personal emissions calculator