In simple term, Clean Development Mechanism or CDM is project activities to reduce global Carbon (CO2) emissions.
It is believed that the global warming is due to the iincrease of global emission of carbon dioxide and what so called
greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide, and ozone, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons). The effect of global warming is the increase of the earth temperature which in turn will make the ices on the North and South Poles melt. In turn it will increase the sea level and decrease the land area of many nations.
One way to reduce emission, based on UN enviromental agreement called Kyoto Protocol, is to make an exchange of what is called emission trading. In this case, a private sector project in that need some funding to reduce emission (usually in developing country), can be assisted by investor in term of financial and technology transfer (usually from industrialized country). Then they have join implementation (JI)