Al-Qaeda, meaning "the base", was created in 1989 as Soviet forces withdrew from Afghanistan and Osama Bin Laden and his colleagues began looking for new jihads.
The organisation grew out of the network of Arab volunteers who had gone to Afghanistan in the 1980s to fight under the banner of Islam against Soviet Communism.
During the anti-Soviet jihad Bin Laden and his fighters received American and Saudi funding. Some analysts believe Bin Laden himself had security training from the CIA.
The "Arab Afghans", as they became known, were battle-hardened and highly motivated.
In the early 1990s Al-Qaeda operated in Sudan. After 1996 its headquarters and about a dozen training camps moved to Afghanistan, where Bin Laden forged a close relationship with the Taleban.
The US campaign in Afghanistan starting in late 2001 dispersed the organisation and drove it underground as its personnel were attacked and its bases and training camps destroyed.
Cells across the world
The organisation is thought to operate in 40 to 50 countries, not only in the Middle East and Asia but in North America and Europe.
In western Europe there have been known or suspected cells in London, Hamburg, Milan and Madrid. These have been important centres for recruitment, fundraising and planning operations.
Attacks attributed to al-Qaeda or associates
• 1993: World Trade Center bombing
• 1996: Killing of 19 US soldiers in Saudi Arabia
• 1998: East African bombings
• 2000: Attack on USS Cole in Yemen
• 2001: Suicide attacks on New York and Washington
• 2002: Attack on Israeli tourists in Mombasa
• 2003: Four simultaneous bomb attacks in Riyadh
For training, the group favours lawless areas where it can operate freely and in secret.
These are believed to have included Somalia, Yemen and Chechnya, as well as mountainous areas of Afghanistan.
There have been reports of a secret training camp on one of the islands of Indonesia.