A paragraph is a series of sentences that are organized and coherent, and are all related to a single topic. Almost every piece of writing you do that is longer than a few sentences should be organized into paragraphs. This is because paragraphs show a reader where the subdivisions of an essay begin and end, and thus help the reader see the organization of the essay and grasp its main points.
Paragraphs can contain many different kinds of information.
A paragraph could contain a series of brief examples or a single long illustration of a general point. Regardless of the kind of information they contain, all paragraphs share certain characteristics. One of the most important of these is a topic sentence. Readers generally look to the first few sentences in a paragraph to determine the subject and perspective of the paragraph. That's why it's often best to put the topic sentence at the very beginning of the paragraph.