Al-Qaeda is an organisation of Islamic militants that has declared "holy war" on Americans, Jews and their allies.
It is blamed for thousands of deaths in the 11 September attacks on the US and other attacks around the world.
It is led by Osama Bin Laden, a millionaire Saudi dissident stripped of his citizenship in 1991.
Experts say it is a loose-knit global network, with links to radical groups in Algeria, Central Asia, Kashmir, the Philippines and across the Middle East.
Al-Qaeda claims to be avenging wrongs committed by Christians and Jews against Muslims over the ages.
It wants to re-shape the Muslim world, replacing secular states with a single Islamic political leadership.
It also wants to drive Americans and other non-Muslims from Saudi Arabia, the home of Islam's holiest sites.
Al-Qaeda draws support from people who see the US's military action in Iraq and Afghanistan, and its support for Israel, as a war against Islam itself.
Al-Qaeda emerged in Afghanistan in the late 1980s, as Arab volunteers joined US-backed Afghan mujahideen fighting to expel the occupying Soviet forces.
Bin Laden set up an organisation to help the volunteers, which became known as al-Qaeda, or "the base".
He left Afghanistan in 1989, returning in 1996 to run military training camps for thousands of foreign Muslims.
In 1998 he called for attacks on US soldiers and civilians. Bombs soon destroyed two US embassies in Africa.
The US-led war in Afghanistan in 2001 toppled the Taleban regime which had given Osama Bin Laden sanctuary.
He is thought to have fled to an area near the Pakistan border. Experts believe he is alive but unwell.
Some high-ranking al-Qaeda figures have been killed and others captured as part of the US-led "war on terror".
Others are still at large. They include Bin Laden's chief strategist, Ayman al-Zawahri, an Egyptian now thought to be in operational control of al-Qaeda.