Which sections go into a dissertation
The acknowledgments is a paragraph that acknowledges (thanks) everyone who has helped you while you have been researching and writing your dissertation. This may be your supervisor or any other academic staff that have provided guidance and support, other students or colleagues that you've collaborated with, interviewees, librarians, or perhaps any external bodies that have given you assistance.
The example shown on this page is taken from one held at the Solent Electronic Archive.
Does every dissertation or thesis have one?
No, not necessarily. It is a good idea, if you are clear about who you should thank - don't just include one because you feel you ought to. Do check previous students' papers in the library and talk to your supervisor about whether to include one.
Where do you put it?
Usually it is the first page after the title page, though some people put it after the abstract. Again, check for the normal practice in your subject.
The next page details the Abstract.