The most obvious part of a thesis proposal, this portion deals with the purpose of your research: the problem it will address. In many cases, the problem is posed as a question, the answer to which should be learned through the course of your research.
In short, this segment is why the problem is significant. It should cover who or what your research will potentially effect, and why information concerning the problem (and potential solutions) is necessary.
Also sometimes called a "Review of Past Literature," this portion of the proposal covers all prior studies done on your topic. This is a very difficult segment, as many students make the unfortunate mistake of selecting proposal topics that have been too well-addressed by existing research, relegating the proposed research unimportant.
The possible approach and methodology is often split into various segments depending on the type of thesis proposal. In more scientific fields, this part may be split into sections called "Design of the Study" and "Analysis of the Data." In any case, this portion of the thesis proposal deals with how research will be conducted, over what terms and how information will be categorized and analyzed.
The potential or expected outcomes of research are often treated as the "summary" or "conclusion" of the thesis proposal. This is the section where the information imparted in all prior sections is drawn together. Drawing points from all prior sections is highly recommended.