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Extremist Sunnis like al Qaeda and its followers "operate in a continuous, mid‐to‐high intensity manner, seeing war against infidels and apostates as a perennial condition, featuring overlapping waves," it says.

But Hezbollah, the Lebanon-based Shiite organization backed by Iran and Syria, concentrates on its open-ended campaign against Israel. Beyond that, Iran-sponsored terrorists take on only "discrete terror campaigns tethered to state and organizational objectives," such as undermining U.S. forces and objectives in Beirut and elsewhere in the region, writes Col. Thomas F. Lynch III in "Sunni and Shi'a Terrorism: Differences that Matter."

The report is an update of a paper Lynch wrote for the Brookings Institution in December, 2008.

Hezbollah can also operate from the shelter of Iran's embassies and consulates around the world for terrorist missions and recruitment, Lynch points out, while al Qaeda and other Sunni extremists have to improvise without the backing of any state. As in Afghanistan, Pakistan, Yemen and Britain, they rely on "coreligionist expatriate communities."

Their hostage-taking styles are different, too, says Lynch, who has served as a military assistant to the U.S. Ambassador to Afghanistan, Zalmay Khalilzad, among other high level assignments

Shiites "have a propensity to kidnap innocents to barter, while Sunni extremists more frequently abduct to kill." Shi'a terror groups also favor "targeted assassinations for specific political gain,"  Lynch says, over the "high‐casualty killings featured in Sunni terrorism," ranging from the massive car bombs that terrorize Iraq today to the 9/11 attacks on New York and the Pentagon.  

For more nuggets and further explication, read the 75-page study itself.