Movements vs Organizations

Juan Cole explains why certain terrorist groups are able to survive assassinations:

I would argue that social movements (as opposed to organizations) are particularly difficult to decapitate. Organizations are characterized by a high degree of integration and are tight systems. Movements are more informally arranged than are organizations, and their flexibility and vagueness can help them withstand attacks on leaders...The Greens in Iran since last summer have been a movement, and it seems obvious that Mir Hosain Mousavi and Mehdi Karroubi as leaders are not all that central to it. The Sadrists in Iraq are a movement, and after a campaign of arrests and assassinations waged against them by the U.S. and British militaries and then the government of Prime Minister Nuri al-Maliki over years, they continue to survive and reemerged to take some 12 percent of seats in the Iraqi parliament on March 7.