The third plane was flown into the Pentagon, which is a large building near Washington, DC that is the headquarters (or most important building) for the U.S. military.
The fourth plane crashed in a field in Pennsylvania, a state that's also in the northeastern part of the country.
Many people think that the hijackers had wanted to crash that plane into the White House where the president lives, but the people on the plane stopped them from doing that.
Almost 3,000 people died in the terrorist attacks that day.
The attacks had a big effect on the economy, too.
Many businesses in the World Trade Center and nearby buildings had to close because they had too much damage and they had lost too many employees.
All airplanes were grounded, meaning that they were not allowed to fly right after the attacks.
The stock market (or the place where ownership in companies is bought and sold) was closed for one week.
But the biggest change was that Americans no longer felt safe.
They felt vulnerable to terrorists, knowing the terrorists could hurt them at any time.
The United States now has many new laws for fighting terrorism, but everyone knows that another attack could happen anywhere and at any time.
In that way, the attacks on September 11, 2001 have changed life in the United States forever.