Abstract is a brief and concise descriptive summary of your thesis or dissertation. It is a sum up of your statement of the problem, hypotheses, assumptions, research design, objectives, data-gathering, data-processing method, findings, conclusions and recommendations. It is brief summary of your thesis outline from the beginning to end. Thesis abstract is place after your table of contents page.
Your abstract should consist of direct quotes which are lifted from the thesis. Your paragraph should use simple language and avoid using technical terms as much as possible. Abstract contains 100 to 350 words only if it is a descriptive type but if it is informative type, it should contain 100 to 250 words only. Also, thesis abstract should focus more on problem and solution discussed in the thesis.
By the way, your thesis abstract is served as great introduction or opening section for the actual thesis. This is for the benefit of your readers because not everyone has to go through the entire thesis; they just want to know important information. Likewise, the examiner board generally does not have enough time to go through the thesis of all the students who shall defend their work in front of them so after reading your thesis abstract, they have the idea about the work done by the student and can ask intelligent questions.
The best way to write your thesis abstract is after the rest of the paper is completed and take note important points. Anyway, how can you summarize something that is not yet written?