What is a “thesis statement”?
Thesis statement is your central idea or argument. It is what you as the writer use to inform readers of what the main focus of the essay is. It is a sentence or two at the end of you're introduction that states your point of view or the argument you’re going to be making throughout the entire essay. It represents what you consider to be the most significant area of your research. The thesis statement isn’t long and drawn out; it’s straight-forward and assertive. It is an argument that can be debated and should be narrow and specific, so that it is easy to manage and develop in to a complete and proper essay. It answers a question that you or your readers may have about a topic.
A thesis statement is not a description, a question, a personal belief, nor is it a generalization. It represents the insight of your article and takes a stand rather than making generalizations or giving opinions.
As with any debate or argument, if you want to be “heard” you must offer a reason to listen. This makes the supporting paragraphs more vital in an essay. Therefore, a thesis statement is not only a means of establishing what the essay is about, but it is also an organizational tool in the developing of your essay. With a working thesis statement you can then decide what information to include and at which point to include the information.
Once you’ve established the topic of your essay and clearly stated it as such, begin using supporting details to elaborate on your thesis. However, if you find in you’re writing, your essay isn’t further elaborating on the thesis you’ve chosen or you're thesis is not being supported, revise to avoid falling short of the expectations readers have in reading your essay, as well as losing credibility. In doing so, you strengthen your essay and your thesis statement now offering better logic and detail. A successful essay has a well thought-out, clarified, precise thesis statement with evidence to support it; leaving no reader confused about what they’ve just read.
A few  examples
See What is a “thesis statement” and how do I write a strong one? for more information.