Al-Shabab is a Somali-based militant group that has ties to al-Qaida and has been waging an insurgency against Somalia's Western-backed transitional government.
Al-Shabab has pledged its allegiance to al-Qaida leader Osama bin Laden, and there are ideological similarities between the groups.
Al-Shabab, which means "The Youth" in Arabic, controls large parts of southern Somalia and the capital, Mogadishu. The group seeks to replace Somalia's transitional government with an Islamic state. It has imposed a strict interpretation of Islamic law in areas that it controls.
The group has carried out suicide attacks against peacekeepers in Somalia in recent years. Sunday's bombings in Uganda marked al-Shabab's first attack outside Somali territory.
The United States and other Western countries designate al-Shabab a terrorist group and U.S. officials have warned that Somalia could be a terrorist breeding ground with its instability and lack of a strong central government.
Neighboring Kenya says it has heightened security along its border with Somalia to guard against an attack.
Somalis accuse al-Shabab fighters of using residents as human shields and launching mortar shells at peacekeepers from densely populated areas.
Some information for this report was provided by AP.