Theory:

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 the monotony.
Important!
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 a heading, 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.
Important!
The paragraphs should start further from the margin than the main part of the paragraph - this is called an indent.
Example:
     People moved slowly then. They ambled across the square, shuffled in and out of the stores around it, took their time about everything. A day was twenty-four hours long but seemed longer. There was no hurry, for there was nowhere to go, nothing to buy and no money to buy it with, nothing to see outside the boundaries of Maycomb County. But it was a time of vague optimism for some of the people: Maycomb County had recently been told that it had nothing to fear but fear itself. – Harper Lee, To Kill a Mockingbird.
 
In the above example, note the following:
  • The paragraph has started a few spaces away from the margin. It is very important to indent a paragraph, to make a clear distinction.
  • The author tries to explain about one idea - the people moving in the place called Maycomb County. The whole paragraph has unity of thought.
  • Note the order of the paragraphs. He has started with an introduction line to say about the people movement. Then he goes on to explain how it happened and how it seemed. Later he introduced the reason. The final line proves to be concluding and also provides a room for curiosity for the next paragraph. It leads to the next thought by arousing interest in the minds of readers.
  • Also, look at the length of the paragraphs before and after the explained paragraph - they are of varying lengths.
Reference:
http://www.kkoworld.com/kitablar/harper_li_masqarachini_oldurmek-eng.pdf