Essays, research papers and other long documents often use the "classic essay format." Documents using this format are divided into three parts: an introduction, a body and a conclusion.
The most important task the introduction serves is to give the reader a preview of the major points your paper will make. It's sometimes a good idea to write the introduction last, after you already know what your paper says about your topic.
Ideally, the introduction should somewhere contain a single sentence that tells, in a nutshell, what major point your paper makes about its topic. Sometimes, it's necessary to spread this nutshell statement, or thesis, over two or three sentences. Think of the introduction as a short telegram that summarizes the paper's content.
For essays, research papers and other long documents, the body is organized into blocks of text called paragraphs. Paragraphs have a formal structure that you should follow. In the body of your paper, each paragraph should start with a one-sentence assertion about some part of your topic. That assertion is called a topic sentence. Its purpose is to tell your reader the paragraph's main point. The body of the paragraph then provides detail necessary to explain or back up the assertion in the topic sentence. The clincher ties the whole paragraph together and often references the topic sentence. The body of most essays and research papers contains several paragraphs, all organized roughly like the one above.
The conclusion should remind the reader of the paper's major point. It can do that by restating, in different words, the nutshell summary of the introduction. The conclusion could also remind the reader of major ideas presented in the body of the paper. The conclusion is your last chance to make sure the paper accomplishes the purpose of the assignment.