"9/11" is shorthand for four coordinated terrorist attacks carried out by al-Qaeda, an Islamist extremist group, that occurred on the morning of September 11, 2001. The attacks killed 2,977 people.
On the morning of September 11, 2001, 19 terrorists from al-Qaeda hijacked four commercial airplanes, deliberately crashing two of the planes into the upper floors of the North and South towers of the World Trade Center complex and a third plane into the Pentagon in Arlington, Va. The Twin Towers ultimately collapsed because of the damage sustained from the impacts and the resulting fires. After learning about the other attacks, passengers on the fourth hijacked plane, Flight 93, fought back, and the plane was crashed into an empty field in western Pennsylvania about 20 minutes by air from Washington, D.C.
The attacks killed nearly 3,000 people from 93 nations. 2,753 people were killed in New York, 184 people were killed at the Pentagon and 40 people were killed on Flight 93.