Given that each viva is necessarily different from all others in some way, you are likely to be called into the examining room by your supervisor or by the internal examiner. Before you arrive, the examiners will have already discussed their opinion of your thesis (and exchanged, where required, their independent reports). They will also have already decided the strategy of the examination (that is, who is going to ask which questions and in what order).
The examiners will then introduce themselves and inform you about the general procedure of the examination. Remember that they will be unlikely to give you any indication about their agreed evaluation of your thesis. Some examiners prefer to tell you the expected result of your viva beforehand but this is not by any means the general practice, so be prepared for either approach, bearing in mind that if they don't tell you it doesn't mean anything about your result one way or the other.
The first question will almost certainly be a very general one, such as
Their training may have encouraged them to start with a simple warm-up question such as this - do not assume that it is a trap: it isn't.
After some general questions, you should expect more detailed questions. Examiners may go so far as to refer to a single statement ("on page x, line y."), asking you to justify/explain or expand on it. You are not supposed to know your thesis by heart, but you still need to remember its general structure and the key points of each section. (You can bring a copy of your thesis with you).
The closing questions may concern the potential development of your (and in your) field of research, that is
As soon as your viva ends, you will be asked to leave the room, so that examiners can freely exchange their views about your exam. The results may be communicated on the same day as the viva, but again this is not the rule and depends on your institution's stipulations and who actually holds the authority to recommend the degree.
Nota bene: Unfortunately it is very common to spend time on minor errors, such as inconsistencies in scholarly presentation. This is a depressing waste of interesting discussion time, since the viva constitutes a rare and precious opportunity to get serious feedback from specialists in your field. Therefore try to get the presentation right so that the viva is not taken up by typos and other minor imperfections.
Granted once again that each viva is unique in its own way, a certain number of questions tends to recur. We can group them into these five sections (what you will find below is a concise version; it is very likely that the examiners will expand these "cores" into a fuller question):
General (opening) questions about contents
General questions about method
Questions concerning one specific aspect of your research
Questions about possible development of your research
Questions about the future development in your area of study
Vivas in Britain and Ireland are generally conducted in English, even if some faculties allow students to submit the thesis in their mother-tongue or in the language of the country to which they devote their thesis (since the vast majority of primary sources - and quotations - are necessarily in that language). If you wish to have your viva conducted in a language other than English, this must be agreed beforehand, and depends on your university regulations. Candidates should in general be prepared to do their viva in English using appropriate registers and terminology.
No academic, however experienced, gives a public lecture, or even the simplest lesson, without notes. There is therefore no reason to attend your viva without your thesis or your notes, in the hope of making a better impression. You are supposed to have carried out your PhD research and to have written your thesis, not to have learned it by heart. You should bring
Do not forget that the large majority of the questions you are asked are real not trick ones and should not represent a problem, so long as you did your thesis yourself, since no one can clarify its foundations and outcomes better than you. Remember at the same time that you are expected to defend your PhD firmly, showing confidence in your achievements, but that you should not be over-self-confident and confrontational. Do not rush your answers, that is
Keep icy calm even if you realize that you are not answering a question as you are supposed to or if you do not know its answer at all: it will not jeopardize your viva. But do not start clutching at straws, be honest: say you cannot answer the question on the spot, but you will immediately delve into that aspect. In the very unlikely case that an examiner finds a real weakness in your thesis, do not aggravate the situation by irritating the entire panel with a flimsy defence: concede the point, acknowledge that it limits the validity of your conclusions but go through it all in detail underlining the aspects which are not affected by the weaknesses they have highlighted. Finally, do not start panicking if they go on firing new questions on the same subject. It does not imply that they are not satisfied with your replies; it is often the result of the examiners' genuine interest in a particular aspect of your research.