19 young adult men - 15 Saudi nationals, 2 Emaritis, 1 Egyptian national, and 1 native of Lebanon - who were associated with the Al-Qaeda terrorist organization, headed (at the time) by Saudi exile Osama bin Laden - violently hijacked four transcontinental flights (two 767s and two 757s, one of each from American Airlines and United Airlines) and crashed three of them, one after the other, into each of the Twin Towers of the World Trade Center in NYC, and the Pentagon in Arlington, VA. The hijackers on the fourth flight - likely intended to crash into the US Capitol building - were successfully prevented from reaching their target by the passengers and crew fighting back.
All on board the four flights were killed, as were well over 2,000 people in NYC from the unbelievable devastation and eventual collapse of the Twin Towers and 125 people working in the Pentagon at the time.
Among the 19 hijackers, the leaders/pilots of the operation had been living in the United States for a few years prior to the attacks. They trained at American flight schools, received financial and operational assistance from other Al-Qaeda operatives working in other countries around the world, and very likely received some additional financing from wealthy individuals from Saudi Arabia and other Persian Gulf states.
Among the many possible motives for the attacks, the ones that I personally find to be the most important for understanding 9/11 is that Osama bin Laden wanted to:
1) Send a message to the Saudi royal family about its relationship with the US government;
2) Send a message to the United States about how much bin Laden and others resented and hated the US for its influence over the Middle East- and by extension, the "Islamic world" as a whole
3)Send a message to the world over how the "Great Satan", the "head of the snake", was a "paper tiger" and couldn't even defend its own people on its own soil.
4) Inspire and encourage other terrorist and militant groups within the Islamic world to fight others
5) Get a lot of media attention, exposure, publicity, etc. and make Al-Qaeda and Osama bin Laden household names.
6) A recruitment effort for what bin Laden (and the Taliban) correctly anticipated would be a "War on Terror" - a recruitment effort for the jihadists.
7) A provocation to the United States to get into wars in the Middle East, South Asia, and potentially elsewhere -which, bin Laden hoped, would turn world opinion (and especially, Islamic world opinion) against the "Great Satan" and its allies.
Al-Qaeda exploited existing vulnerabilities within America's national security apparatus. By carrying out the attacks of 9/11, they demonstrated that a relatively small cadre of determined, savvy, and intelligent fanatics could inflict a lot of damage, in a sensational and shocking way, and that they could change the course of history in ways that all of us are still grappling with.