The environmental movement is made up of radicals, idealists, and realists. Let's briefly examine each type:
Radicals seek fundamental change of the system. They believe environmental goals cannot be realized without deep socio-economic-political changes, and thus tend to be anti-corporate. Winning individual short-term battles is less important to them than changing the world in the long term. Many feel that the ends justify the means. The best radicals suppress emotion to implement their strategy.
Idealists are usually altruistic. They view the world from a very moral and/or ethical perspective, with individual responsibility and example paramount. They are emotionally involved and believe the ends never justify the means.
Realists view the world as a poker game--the cards are dealt and you do the best you can with your hand. Their actions focus on the short term. Although they believe the ends can often justify the means, they prefer to work within the system. They can live with trade-offs and do not seek radical change, if for no other reason than they see it as unobtainable.
Some examples may help to clarify these categories. Earth First! was founded by radicals and is now dominated by idealists. The Sierra Club has a membership of idealists and a staff of realists. Greenpeace is idealistic with some radical tendencies but not to the extent of the Sea Shepherd Society.
To stretch the “boat-rocking” analogy, realists want to help steer the boat, however small the change of course; idealists would rather the boat not move at all if it doesn't turn far enough in the right direction; and radicals would just as soon capsize the boat."
From Andy Kerr
See also a more in-depth analysis of activist types.