What was the target of United Airlines Flight 93?

Every defense mechanism the U.S. government had on 9/11 failed except for one: the passengers and crew aboard United Airlines Flight 93. They were on the final plane to be hijacked that day and heard reports of the attacks from loved ones on the ground. Then they rallied to stop the terrorist plot.But where was the plane headed before it crashed? What was the intended target of the Flight 93 hijackers?