Topic Sentence :
A topic sentence is a sentence whose main idea or claim controls the rest of the paragraph; the body of a paragraph explains, develops or supports with evidence the topic sentence's main idea or claim. The topic sentence is usually the first sentence of a paragraph, but not necessarily. It may come, for example, after a transition sentence; it may even come at the end of a paragraph.
Topic sentences are not the only way to organize a paragraph, and not all paragraphs need a topic sentence. For example, paragraphs that describe, narrate, or detail the steps in an experiment do not usually need topic sentences. Topic sentences are useful, however, in paragraphs that analyze and argue. Topic sentences are particularly useful for writers who have difficulty developing focused, unified paragraphs (i.e., writers who tend to sprawl). Topic sentences help these writers develop a main idea or claim for their paragraphs, and, perhaps most importantly, they help these writers stay focused and keep paragraphs manageable.
Topic sentences are also useful to readers because they guide them through sometimes complex arguments. Many well-known, experienced writers effectively use topic sentences to bridge between paragraphs. Here's an example of how one professional writer does this:
Soon after the spraying had ended there were unmistakable signs that all was not well. Within two days dead and dying fish, including many young salmon, were found along the banks of the stream. Brook trout also appeared among the dead fish, and along the roads and in the woods birds were dying. All the life of the stream was stilled. Before the spraying there had been a rich assortment of the water life that forms the food of salmon and trout — caddis fly larvae, living in loosely fitting protective cases of leaves, stems or gravel cemented together with saliva, stonefly nymphs clinging to rocks in the swirling currents, and the wormlike larvae of blackflies edging the stones under riffles or where the stream spills over steeply slanting rocks. But now the stream insects were dead, killed by DDT, and there was nothing for a young salmon to eat.
Rachel Carson, Silent Spring
The first part of Carson's topic sentence — Soon after the spraying had ended — is a transitional clause that looks back to the previous topic: DDT spraying. Topic sentences often begin with such transitional clauses referring to the previous paragraph. The second part of the topic sentence — there were unmistakable signs that all was not well — shapes and controls what follows. This kind of bridging helps the reader follow Carson's argument. Notice, too, how Carson further helps the reader follow her argument by providing a more focused version of the topic sentence later in the paragraph — All the life of the stream was stilled. This sentence tells us exactly what Carson meant by all was not well.