If you want to be a good writer, you should know what paragraph is. So, what is a paragraph? You are right. A paragraph is a group of sentences that explain or expresses one idea. All of the sentences in a paragraph have to talk about one idea. This idea is called the topic. The main idea of a paragraph usually consists of the topic and the controlling idea (the opinion of the writer).
A paragraph normally has three parts.
Let's see those three parts of a paragraph now. First, the topic sentence usually consists of two parts, the topic and the controlling idea ( or the opinion of the writer ). Second, the supporting sentences usually give details, reasons and explanations about the topic sentence. Last, the concluding sentence usually restates the topic sentence in a different way. See the following example :