A green roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems. (The use of "green" refers to the growing trend of environmentalism and does not refer to roofs which are merely colored green, as with green roof tiles or roof shingles.)
Container gardens on roofs, where plants are maintained in pots or trays, are not generally considered to be true green roofs, although this is an area of debate. Rooftop ponds are another form of green roofs which are used to treat greywater.

Also known as "living roofs", green roofs serve several purposes for a building, such as absorbing rainwater, providing insulation, creating a habitat for wildlife, and helping to lower urban air temperatures and combat the heat island effect. There are two types of green roofs: intensive roofs, which are thicker and can support a wider variety of plants but are heavier and require more maintenance, and extensive roofs, which are covered in a light layer of vegetation and are lighter than an intensive green roof.
The term green roof may also be used to indicate roofs that use some form of green technology, such as a cool, light colored roof, a roof with solar thermal collectors or photovoltaic panels. Green roofs are also referred to as eco-roofs, vegetated roofs, living roofs and greenroof.

Environmental benefits:

• Green roofs are used to reduce heating (by adding mass and thermal resistance value)
• A 2005 study by Brad Bass of the University of Toronto showed that green roofs can also reduce heat loss and energy consumption in winter conditions.
• Reduce cooling (by evaporative cooling) loads on a building by fifty to ninety percent especially if it is glassed in so as to act as a terrarium and passive solar heat reservoir — a concentration of green roofs in an urban area can even reduce the city's average temperatures during the summer
• Reduce storm water run off
• Natural Habitat Creation
• Filter pollutants and carbon dioxide out of the air which helps lower disease rates such as asthma
• Filter pollutants and heavy metals out of rainwater
• Help to insulate a building for sound; the soil helps to block lower frequencies and the plants block higher frequencies
• If installed correctly many living roofs can contribute to LEED points
• Increase agricultural space
• Financial benefits
• Increase roof life span dramatically
• Increase real estate value
• A green roof is often a key component of an autonomous building.

Several studies have been carried out in Germany since the 1970s. Berlin is one of the most important centers of green roof research in Germany. Particularly in the last 10 years, much more research has begun. About ten green roof research centers exist in the US and activities exist in about 40 countries. In a recent study on the impacts of green infrastructure, in particular green roofs in the Greater Manchester area, researchers found that adding green roofs can help keep temperatures down, particularly in urban areas: "adding green roofs to all buildings can have a dramatic effect on maximum surface temperatures, keeping temperatures below the 1961-1990 current form case for all time periods and emissions scenarios. Roof greening makes the biggest difference…where the building proportion is high and the evaporative fraction is low. Thus, the largest difference was made in the town centers."

Thanks to Wikipedia for most of this information.