Now that you got to this level, you are pretty sure to receive an approval for your thesis. Professors don't allow their students to present the thesis unless they feel confident about it since it represents the quality of their work too.
Unlike the seminar, the defense is usually formal, with a number of
professors attending and maybe no friends at all. However, you should not
fear such a situation because you already made a trial for it on the
seminar day.
The defense differs from the seminar also in a ver important manner from
the technical point of view: you must submit pseudo-final copies of your
thesis to the members of the committee some time before the defense day
(maybe a week or two). They read it and come up with strange questions
about everything you did, so be prepared for harder questions and deeper
discussions. They may even challenge you with new proposals and ask you to
evaluate them to see how do you think and analyze previously unknown
situations.
However, remember what I just said. If you are not good enough, your professor will not let you do it from the first place. So keep your calmness and try to answer. If you don't know the answer, propose a strategy that might help to find the answer. The important thing is not to stand stairing at them and waiting for the answer to drop on you from heaven (it will not happen!)
Once they finish the "torture" phase they will start to be friendly and may even smile if you did a good job. They may tell you to step out of the room so that they discuss their decision. Go out and drink a lemonade to cool down. When you are called back to the room, listen to any suggestions/additions they propose to enhace your work and thank them for it.
That's it. You have done it. This is your glory day after a long time of slavery. Enjoy.