Al-Shabab -- meaning "The Youth" in Arabic -- is the largest group among several armed Somali organizations that were started with the goal of toppling Somalia's U.N.-backed Transitional Federal Government and imposing Islamic law.
While the members of al-Shabab hold diverse viewpoints, some in the group believe al-Shabab's mission is not only to overthrow the Somali government, but also to carry out global jihad. In recent years, some factions of the group have become more radical and aligned themselves with al-Qaeda.
Members have been recruited in several countries including the U.S. In 2009, U.S. authorities indicted 14 young Minnesotans who were believed to have joined al-Shabab. In September of this year, the group claimed responsibility for killing Omar Hammami, a native of Daphne, Ala., who had moved to Somalia to join the group but then had a falling-out with other members.
Al-Shabab has claimed responsibility for many other attacks, including the coordinated shooting rampage on Sept. 21, in an upscale mall in Nairobi, Kenya that left at least 39 people dead. It was also behind a 2010 attack in Kampala, Uganda, during which 74 people were killed while watching a World Cup match.
The group is targeting Kenya and Uganda primarily because troops from the two countries provide support to the Somali Transitional Federal Government.
While al-Shabab has turned into a largely violent organization, for a time it was run as a counter force to criminal gangs operating in Mogadishu, Somalia's capital. Al-Shabab was once the military wing of the deposed Islamic Court Union (ICU), which controlled much of central and southern Somalia in late 2006.
But Al-Shabab's fighters were eventually forced out of Somalia by Ethiopian troops in support of the largely powerless U.N.-backed interim government.
Though the group has carried out attacks in other countries, it has mostly focused on attacks within Somalia, using suicide bombs to kill dozens over the years. Its members have also assassinated international aid workers and others perceived to be friendly to Somalia's transitional government.
Al-Shabab is regarded by the U.S. and many other governments as a terrorist organization.
While it is not clear how al-Shabab funds its activities, Eritrea and some Arab nations have been accused of providing the organization with financial and logistical support.