One of my favourite aspects of academia in the UK is the final oral examination for the PhD --- formally called a viva voce, which everyone seems to call a viva (VEYE-vah). The viva is an oral examination that typically consists of the student and two examiners, one from within the University (the internal examiner), and one from outwith the university (the external examiner).
Both examiners read the thesis carefully, and ask the student detailed questions. The traditional way to do this is that all three have a paper copy of the thesis, and the examiners go through the thesis page by page with their questions. Some questions are high level ("Why did you choose technique X rather than technique Y?") and some can be very detailed ("In your proof of Theorem 4.3 on page 176, I'm not sure that the third step is correct. What if the matrix A is non-singular?").
This discussion commonly takes 2-3 hours. Longer and shorter vivas are not unheard of, though if you ask me, a one-hour viva is a bit of a rip-off for the student, and a five-hour viva isn't kind, unless the length is caused by the student being exceptionally argumentative or loquacious.
At the end of the viva, the student is asked to wait outside, and the examiners decide whether the student should be awarded a PhD, and if so what corrections to the thesis are required. The most common outcome is a pass subject to minor corrections, which the student is allowed a few months to complete.
Essentially, in my experience a viva is a detailed technical discussion of the content of the thesis. Most students start out nervous, sometimes exceedingly so, but relax after a few questions as they realize that this is just a technical discussion of the sort that they have had many times before. That said, even if your research career is long, it is rare that a trusted colleague will provide you with several hours of detailed feedback on your work. To be part of a discussion like this, on either side of the table, is a privilege: most work is ignored, so any criticism is a compliment.
I am sure that the process is more tense if the examiners believe that the quality of the thesis is borderline --- fortunately, I haven't yet been asked to examine a thesis like that. If portions of your thesis have already been published in prestigious venues, and whether this is possible at all varies greatly across disciplines, then you can be fairly sure that your thesis is not near the borderline.
A colleague suggested to me once that a viva is like a negotiation. If your thesis represents a sufficient amount of research of acceptable quality (and if it was indeed you that wrote it), then you will pass. The negotiation is over which corrections will be required. Responsible examiners do not want to require additional experiments that will require months of work, if the thesis as submitted is of excellent quality. But they also don't want a thesis to be passed with gaping holes in its argumentation. The purpose of the discussion is to sort out which potential concerns are which, and your voice in this discussion matters --- you wrote the thesis, so you are the expert in the room.
Sometimes an examiner asks a question with an eye to a correction being required. Maybe you think, "yeah, that's a good point, I should add a paragraph on that" --- in this case, don't hesitate to say so. On the other hand, if providing a good answer to the examiner's question would require months of additional research, politely explain why, while also giving the best answer you can given what you do know. What you don't want to do is argue every point strongly, even when the examiners are clearly right... that is not good negotiation strategy.
A bit more about who attends the viva. The internal examiner is not the supervisor, in fact, they will not have been involved with the thesis research at all. It is not unusual for the internal examiner to be a bit of generalist with respect to the thesis topic, although when I have served as internal, I have usually been able to make out the thesis reasonably well. Presumably this is because the School of Informatics is large enough that there are many theses in machine learning and natural language processing that need to be examined.
The external examiner is chosen specifically for their expertise in the subject matter, and to serve as an external is generally seen as a minor indicator of prestige. A certain amount of deference is paid to the external in the culture of the process. Even so, I have the sense that part of the role of the internal is to be accountable to the University (for following correct procedures) and to the student and the supervisor (to make sure that the examination is fair to the student). I have read and heard horror stories of aggressive external examiners but never witnessed one; to the contrary, the examiners who I have witnessed have all gone out of their way to be kind to the student.
The role of the supervisor in the viva, I think that this may vary slightly across institutions. At Edinburgh, the supervisor is allowed to attend the viva, if the student permits, but not to participate in any way. In my experience as internal examiner, the supervisor has attended about half the time. One supervisor silently took notes to share with the student, which I think is quite a kind thing to do.
Finally, although the description is written for a US audience, British academia also fetaures the snake fight portion of your viva.
(Written in honour of my first PhD student graduating. Congratulations Yichuan!)