Al-Shabab means The Youth in Arabic.
It emerged as the radical youth wing of Somalia’s now-defunct Union of Islamic Courts, which controlled Mogadishu in 2006, before being forced out by Ethiopian forces.
There are numerous reports of foreign jihadists going to Somalia to help al-Shabab, from neighbouring countries, as well as the US and Europe.
It is banned as a terrorist group by both the US and the UK and is believed to have between 7,000 and 9,000 fighters.
Al-Shabab advocates the Saudi-inspired Wahhabi version of Islam, while most Somalis are Sufis.
It has imposed a strict version of Sharia in areas under its control, including stoning to death women accused of adultery and amputating the hands of thieves.
Below we take a look at the major attacks carried out by the group in Kenya
2013: In September 2013, al-Shabab fighters stormed Nairobi’s Westgate Mall, firing indiscriminately at shoppers and killing 67 people in a siege that lasted 80 hours.
The assault horrified the world and exposed weaknesses in Kenya’s security forces.
2014: In November 2014, members of Somalia’s al-Shabab armed group hijacked a bus in Kenya and killed 28 non-Muslims on board.
“What happened in Mandera today we did in revenge for what the non-believer government has done to innocent Muslims,” Sheikh Ali Mohamud Rage, a spokesman for the group, told Al Jazeera.
April 2015: Al-Shabab launched an assault on Garissa University College in Kenya, killing 148, mainly students.
January 2016: Fighters from the Somali armed group assaulted a Kenyan-run military base for African Union peacekeepers, killing scores of Kenyan soldiers.
January 2019: Two explosions and gunfire heard at an upscale hotel complex in Nairobi, Kenya’s capital , the aftermath left 14 dead.
The attack on the Dusit hotel complex – which also houses offices and banks – sent people fleeing for their lives.
There could still be armed assailants in the building and the police operation was ongoing, reporters were told in a short briefing.