Leadership: ?
The organization may simply be an a.k.a. of Ansar al-Islam -- indeeed the U.S. Patterns of Global Terrorism lists the name as just that, another name for Ansar al-Islam.  The problem is that Ansar al-Islam is a kurdish region active group, whereas Ansar al-Sunna is thought to be more active in the Sunni populated areas of Iraq which extend well into southern Iraq. In a September 2004 video, the goals of the group are iterated as:  "The goal is not only to get rid of the occupiers of Iraq, but to establish an Islamic state"
Activities At this time it is not known if there are specific Ansar al-Sunna activities attirbutable to AS or if they are actually simply Ansar al-Islam activities.  Wikipedia states that the group is primarily active in the northern and central regions of Iraq and has strong ties to Ansar al-Islam.  A September 2004 video tape claims credit for the ambush and killing of 7 Spanish soldiers on November 11, 2003, and includes images of several ID documents from the soldiers.
Strength:  Unknown
Links:  Al-Qaeda

Spokesperson speaking on video released by the group in September of 2004.