From Envirowiki

NIMBY stands for Not In My BackYard, and generally refers to activists of any part of the political spectrum, who only act on issues if it directly effects them, and take no action on similar issues on a wider scale. The prime example would be someone attempting to stop a corporation from building a development because it would damage their creek, or ruin their view, being happy to let the same thing happen elsewhere, or even passively supporting the destruction elsewhere, by buying products from those developments, or shipping their waste to that area. Most people are NIMBYs to some extent, a good example of this is a large proportion (although perhaps not a majority) of people in Australia supporting nuclear power, but crying out when it is suggested that they have a nuclear power station in their local area.

NIMBYism in terms of environmentalism can range from quite helpful, to downright destructive. While NIMBYs often act in favor of the environment, they are, by defintion, unaware of, or choose not to act on the broader issues. This can cause the development to be forced into a less suitable area, or could indirectly cause a worse development to occur (the Cape Wind Project is probably a good example of this, NIMBYs crying out about the local impact of a large wind development, but refusing to acknowledge the fact that NOT having it might cause more fossil fuels to be extracted, causing local damage to the mining area, and huge damage to the climate)