The thesis statement is the central or controlling idea in a paper that explains the purpose of the paper, supports an idea, or advances a position.
If a thesis is firm and clear, it may immediately indicate the best way to organize supporting material and eliminate the need for elaborate planning. A thesis must have something to say about a subject or to present a particular viewpoint. The rest of the paper breaks down the thesis and proves it persuasively.
A thesis is different from a topic, which is a general subject area. For example, education is a general subject area that should be narrowed to a more manageable topic. Studying for the SATs narrows the broader subject area education. Brainstorming ideas about a subject helps students to come up with a topic and to formulate a "working" thesis.
A thesis for short essays may be formulated in one sentence. For longer research papers, it may take several sentences to state the thesis.