A thesis statement is a sentence or sentences which summarize the main idea(s) of your paper and clearly expresses what it is you are going to say about your topic. Your statement identifies what topic will be discussed and the purpose of your paper.
The thesis statement:
covers exactly the topic you want to talk about, no more and no less.
lets your readers know what to expect.
usually appears in your paper's introductory paragraph.
helps you better organize and develop the content of your paper.
The rest of your paper presents information you have gathered, organized and developed to support your point of view, explanation or analysis of the topic.