Slide 45 of 61
This is one of the so called 'flexibility mechanisms' designed to help rich (annexe 1) countries meet their Kyoto commitment using methods other than directly via cuts in their own emissions. The flexibility mechanisms have caused some of the biggest arguments about the Kyoto protocol, but it is generally agreed that without them the size of the agreed reduction targets would have had to be much smaller.
Annexe 1 countries can transfer 'emissions units' between them and implement joint initiatives to curb GHG emissions. This kind of joint implementation could involve a co-ordinated forest management scheme for instance. However, there is concern that some of the richer countries may use the JI to bypass cutting their own domestic emissions by claiming the 'emissions credits' from schemes they implement in 'transition' annexe 1 countries like Russia