The dissertation defense is entirely different from your PhD field exams. At a minimum we want to be assured that you can succinctly explain orally the central argument of your thesis and explain its contribution to the literature. You will do this in part with your opening statement but mostly in the process of responding to the various questions or suggestions posed about the thesis.
At its best, however, the defense is a conversation about where the work goes from here--both in the short-term (before a final copy is submitted for graduation) and in the long-term (the eventual revisions for a book). You therefore should not look upon this primarily as a "test". The dissertation defense is scheduled only after your dissertation has been approved—you can't fail unless you faint dead away or become completely tongue-tied. Rather it is an opportunity to get some of the most searching constructive examination of the thesis you'll see prior to readers' reports for a press. It is also an opportunity for you to pull back and see the dissertation as a whole rather than in its component parts.
The general order of the defense (although you should, of course, check with your dissertation chair) is that you will be invited to make an opening statement, which should run from between 10 and 20 minutes (you can write this out, but a list of talking points would be best), and then the committee members will ask questions or make comments. After the discussion is over you will be asked to leave the room while the committee decides on the advice to give you, what will still be required of you, and whether or not the dissertation will receive “Distinction” (approximately 10%) of dissertations. Distinction depends on the quality of the dissertation and of the defense. You may be asked to do no revisions, minor revisions, or major revisions.
The content of your opening statement is up to you, but you should realize that here is your opportunity to shape the entire defense as you wish it to go and to best benefit you. Some people begin by describing "where the project came from," that is the personal or intellectual experiences and/or curiosities/ambitions that suggested the topic to them. One might also try to situate the topic within some historiographic tradition--as a departure from or development of this or that. The main thrust, however, should be forward-looking: where do you go from here? Are there still problems you are wrestling with in the analysis, organization, style? If this is to be a contribution to both the social history of immigration to France and to the history of Jews in France, what should be it chronological frame (e.g., the Third Republic to 1974 or from the first big wave of immigration from Eastern Europe in 1880s to the influx from North Africa after Algerian independence in 1962)? How conceptually does one best integrate fiction, film or material culture into a history dissertation? These are offered, of course, as just examples of what you might want to have us talk about. They are intended illustrate how you can frame the conversation to your ends.