An international agreement linked to the environmental treaty United Nations Framework Convention on Climate Change (UNFCCC), containing legally binding commitments to reduce emissions of greenhouse gases.
According to the UNFCCC the objective is to achieve "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic [man-made] interference with the climate system. Such a level should be achieved within a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner."
The Kyoto Protocol was negotiated in Kyoto, Japan in December 1997, and entered into force in February 2005. So far 183 'Parties of the Convention' (nations and organizations that have signed the Convention), have officially accepted the Kyoto Protocol.
While the Convention only encourage the industrialized countries to stabilize their emissions of greenhouse gases, the Kyoto Protocol commits them to do so.
The Convention and the Protocol divide the responsibilities among the countries according to their status. Since industrialized countries have contributed most to greenhouse gas emissions, they are required to collectively reduce emissions of important greenhouse gases by at least 5 percent (compared to the 1990 levels) over the 2008-2012 period. No such specific limitations are listed for developing countries, but they too agree to the shared responsibility of reducing emissions.
To help the countries reach their goals of reduced emissions, the Protocol establishes some mechanisms:
The limitations in emissions for industrialized countries are expressed as allowed levels of emissions or "assigned amount units" for each country over the 2008-2012 period. If a country has emission units to spare, i.e. has not emitted as much greenhouse gas as it was allowed, it can sell these units to another country. Thereby countries that are above their emissions limit can buy extra emission allowances from other industrialized countries.
In addition to this so-called "emissions trading" between the industrialized countries, they can also invest in emissions-reducing projects in developing countries and receive emission credits in return.
The purpose of both these mechanisms is to aid the reduction of greenhouse gas emissions, while at the same time making them as cost efficient as possible.