19 March Washington Post - Iraq's Insurgents: Who's Who by Jonatahn Finer.

Long considered a fragmentary and disorganized collection of groups with varying tactics and aims, Iraq's insurgency is showing signs of increasing coordination, consolidation and confidence, those who study it now say. There is no consensus on the precise number of insurgent fighters, but estimates range from a few thousand to more than 50,000. The vast majority of insurgents, probably more than 90 percent, are believed to be Iraqis from the Sunni minority group that largely ruled the country before the fall of Saddam Hussein. But U.S. commanders say that most of the deadliest attacks, and particularly suicide attacks, are committed by foreigners from a range of neighboring countries, including Jordan, Syrian, Saudi Arabia, Egypt and Sudan...
Finer categorizes the insurgents into larger and smaller groupings.