How were the September 11 terrorist attacks unique? How were they similar to other terrorist incidents?

The September 11 attacks were unique because they were huge attacks by a foreign power on American soil. This is unusual, because terrorist attacks in the US usually rather have an internal motivation and are never this big - no other attack has claimed so many victims.

It was similar because the goal of the attack was to spread terror - there were no demands made, only violence meant to intimidate.