Al-Shabaab is an Islamist group founded in 2006 and active in Somalia and Kenya as well as Yemen.
The group's name means "The Youth" in Arabic.
It has been designated a terrorist organisation by several governments, including the US, UK, Malaysia, and UAE.
The US State Department also has open bounties out on several senior commanders, including one for $6million (£4.5million) on leader Ahmad Umar.
The latest estimates suggests the group has between 7,000 and 9,000 troops.
The group's militants have been responsible for several deadly attacks in recent years.
In October 2017, two truck bombings in Mogadishu, one of which caused the collapse of a hotel, killed at least 587 and injured more than 300.
The attack was the deadliest in Somalia's history.
The group was also responsible for a gun attack on the Westgate shopping centre in Nairobi, Kenya, in which 67 people died.