A paragraph usually tells about one topic. Often one sentence is the “topic sentence.” It tells the main idea of paragraph. The topic sentence is often-but not always-the first sentence of the paragraph. The other sentences usually give detail or examples to illustrate the main idea.
example : the are several different kinds of classes on university campuses. Professors usually teach undergraduate classes. They give formal lectures. Student have to listen and take notes. Then teaching assistants (T.A.s) lead discussion groups. In graduate seminars, small group of students discuss their ideas with their instructor and classmates.
In this example, the first sentence is the topic sentence of the paragraph. It gives the main idea that there are several different kinds of classes on university campuses. The other sentences give examples of these kinds of classes.