You'll remember that the first step of the reading process, previewing, allows you to get a big-picture view of the document you're reading. This way, you can begin to understand the structure of the overall text.
A later step in the reading process, summarizing, allows you to encapsulate what a paragraph, section, or the whole document is about. When summarizing individual paragraphs, it's likely that your summary ends up looking like a paraphrase of that paragraph's topic sentence.
A paragraph is composed of multiple sentences focused on a single, clearly-defined topic. There should be exactly one main idea per paragraph, so whenever an author moves on to a new idea, he or she will start a new paragraph. For example, this paragraph defines what a paragraph is, and now we will start a new paragraph to deal with a new idea: how a paragraph is structured.
Paragraphs are actually organized much like persuasive papers are. Just like a paper has a thesis statement followed by a body of supportive evidence, paragraphs have a topic sentence followed by several sentences of support or explanation. If you look at this paragraph, for example, you will see that it starts with a clear topic sentence letting you know that paragraphs follow a structure similar to that of papers. The next sentence explains how a paragraph is like a paper, and then two more sentences show how this paragraph follows that structure. All of these sentences are clearly connected to the main idea.
The topic sentence of a paragraph serves two purposes: first, it lets readers know what the paragraph is going to be about; second, it highlights the connection between the present paragraph and the one that came before. The topic sentence of this paragraph explains to a reader what a topic sentence does, fulfilling the first function. It also tells you that this paragraph is going to talk about one particular aspect of the previous paragraph’s main idea: we are now moving from the general structure of the paragraph to the particular role of the topic sentence.
After the topic sentence introduces the main idea, the remainder of the sentences in a paragraph should support or explain this topic. These additional sentences might detail the author's position on the topic. They might also provide examples, statistics, or other evidence to support that position. At the end of the paragraph, the author may include some sort of conclusion or a transition that sets up the next idea he or she will be discussing (for example, you can see this clearly in the last sentence of the third paragraph).