Al-Qaeda, meaning the base, is the organisation or network established and led by Osama Bin Laden.
US investigators say it has a command-and-control centre which discusses and approves major undertakings and attacks. They say it was based in Peshawar from 1989 to about 1991, and then moved to Sudan until about 1996, before relocating to Afghanistan where it operated training camps.
It is believed to form loose operational alliances across continents with a wide range of similarly-minded groups.
Al-Qaeda was founded in 1988 by Bin Laden and Mohammed Atef - believed to have been the organisation's military commander.
It was established as an umbrella group for Islamic militants who fought in Afghanistan against the Soviet Army.
Al-Qaeda is dedicated to opposing non-Islamic governments by any means, including force. One of the organisation's principle aims is to drive US forces from Saudi Arabia - land considered by the group to be holy.
By 1998 it had merged with Egyptian Islamic Jihad, vastly enhancing Bin Laden's reach and organisational ability, according to trial testimonies of those arrested.
Intelligence agencies say al-Qaeda planned and carried out the atrocities on 11 September 2001.
They say the detailed planning was carried out by one of Osama Bin Laden's close associates, who remains unnamed.
They believe al-Qaeda operates in more than 50 countries - in Europe and North America, as well as the Middle East and Asia.
Al-Qaeda has been blamed for the attacks on the US embassies in East Africa in 1998, as well as the killing of 18 US soldiers in Somalia in 1993, and were linked to the attack on the USS Cole in October 2000.
The FBI says it has established that three of the 19 suspected hijackers were associates of al-Qaeda.
One of them has been identified as playing a key role in both the East Africa embassy and USS Cole attacks.
Investigators say the planning and execution of the attacks on 11 September were "entirely consistent with previous attacks", citing multiple simultaneous attacks, the use of suicide bombers, and meticulous long-term planning as examples.
Al-Qaeda regards US civilians as legitimate targets, and investigators say associates of Bin Laden were warned to return to Afghanistan from other parts of the world by 10 September, and were naming the "date for action" as on or around 11 September.
However, due to the shadowy nature and loose organisational structure of al-Qaeda, proving a direct link and tracking the suspects is a complex task.