There are people that your dissertation has to pass through before it will be accepted towards your degree. When you have written a dissertation that is the best you can do, it still has to go through a few hoops to be qualified. During the writing of your dissertation, you were probably prepared for you dissertation defense by your advisor. Since your advisor should have been through one for his or her dissertation, they will know how to prepare you for it. The writing of the thesis or dissertation is a lot of work on its own, and some students can be frightened about doing a defense for their work. It does differ from school to school what you will be up against, but for the most part you will pass your defense. Even though it may seem like a painful experience to some, failed defenses are very rare. The purpose of the defense is actually not just whether the committee approves or not.
Preparing for your defense might take some time and effort, but it is worth it to know what you are walking into. By the time you arrive at your scheduled defense, it will be all up to you. The practice of universities awarding doctorates has been an educational tradition and milestone for longer than the United States has been proclaimed its own nation. Despite that, only students of the past 200 or so years have had to write and defend dissertations. In 19th century Europe, there would often be crowds of people, both academic and public, who would come to listen to a dissertation defense. These days, it is limited to graduate students and faculty members in your department. Do not be surprised if you get there and it is only you and the committee.