Writing a thesis statement for an essay is different than writing an essay about a general topic. For instance the topic "women writers" would be considered a general one. However, a statement that the prose of writer Katherine Mansfield transformed the short story form by introducing a new style of writing to it, however, would be considered a thesis statement. It is an assertion about the work of a particular woman writer that has to be explained and supported with evidence. In a thesis statement, you state the detailed claim that you will argue for in the essay. The thesis statement usually appears at the end of the introductory first paragraph of the essay.
A topic sentence sums up a paragraph within an essay in the same way that the thesis sums up the entire essay. It is used as the first sentence for every paragraph that offers new or additional support for the thesis. Like offering an exhibit at a trial, each topic sentence represents the introduction of another piece of evidence that will prove the thesis. The remainder of the paragraph then explains the evidence and how it tends to confirm the thesis.
Just as each paragraph in the essay is used to support the thesis, each sentence in the paragraph is used to support the topic sentence with relevant evidence or examples. For example, if a topic sentence asserts that Mansfield's prose is markedly different from that of past short story writers and her contemporaries, passages from her work could be cited in contrast to those of other writers on the same themes. Supporting sentences might also explain the literary significance of these differences.
Topic sentences can also be used to reiterate the thesis and its supporting material. In concluding the essay, the writer can highlight the evidence for the thesis by restating the topic sentences and some of the key arguments. This can also serve to unify the individual points of the essay. When the thesis statement is combined with a summary of the supporting facts in the essay's conclusion, it increases its power to persuade the reader of its assertion.