Al-Qaeda, means 'the base'.
The organisation was set up by a man called Osama Bin Laden in 1988.
Many volunteers from Arab countries had gone to Afghanistan during the 1980s. They wanted to help the Afghans fight in a conflict against a place called the Soviet Union.
The volunteers supported the Afghans as they were also followers of Islam. For them, the battle was a Jihad or holy war.
When the conflict was over, al-Qaeda was set up to continue the jihad against people they considered to be 'enemies' of their religion - Islam.
Al-Qaeda is thought to operate in 40 to 50 countries around the world. It is classed as a terrorist organisation by the UK government.