A group of sentences formed together related to one particular topic, or develops to a single point, is called a paragraph.
A paragraph, in simple words, is a group of sentences. Take any book for example, the text is divided into units called paragraphs to make the reading easier and interesting. Also, each paragraph starts with an indent to the right. It is done to show that a new idea or concept is being introduced in the next paragraph.
Why is a paragraph needed?
  1. To make reading easier.
  2. To mark an introduction of a new concept.
  3. To mark a step in the development of an argument.
  4. To reduce monotony.
There is no specific unit to specify the length of a paragraph. It can be short or long, as required to explain the point.
What are the important features of a paragraph?
1. Unity: Each paragraph should deal with a single idea or thought. While writing an essay, each paragraph should have aheading, and a sub-heading if required. Every sentence should be coherent and should develop the idea as it progresses. It must remain closely connected to the main idea of the paragraph.
2. Order: There should be a logical sequence followed in the expression of ideas. The topic should be relevant to the paragraph and vie-versa. There can be development of the topical idea but not a deviation. If there is a counter-argument point, it must be taken to the next paragraph. It should follow the logic of introduction in the first sentence, explanation in the middle part and the conclusion must answer all the questions raised at the start of the paragraph.
3. Variety: The paragraphs can be of varying lengths, as the case requires. It can be of one or two sentences, or even of eight sentences. Varying lengths of paragraphs add variety to the text and make it look better and easier to read.
The paragraphs should start further from the margin than the main part of the paragraph - this is called an indent.
The following is an extract from the lesson. They are the first and second paragraphs of the lesson. When you look into it, you can see the following paragraphs have the characteristics mentioned above.
     I had heard a great deal about Miss Beam’s school, but not till last week did the chance come to visit it.
     When I arrived there was no one in sight but a girl of about twelve. Her eyes were covered with a bandage and she was being led carefully between the flower-beds by a little boy, who was about four years younger. She stopped, and it looked like she asked him who had come. He seemed to be describing me to her. Then they passed on.
National Council of Educational Research and Training (2006). Honeysuckle. A Different Kind of School - E. V. Lucas (pp. 56-62). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.