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. If you are 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.
The example of a paragraph writing from the lesson "The Friendly Mongoose":
  
     One day, the farmer's wife had to go out to buy groceries. But she didn't want to leave the baby alone with the mongoose. So she asked the farmer to look after the baby till she arrives. But the farmer felt bored and went out to take a look at the fields. On his way, he met some of his friends.

     When the wife got back home, she saw the mongoose's face smudged with blood. She screamed, believing that it had harmed her baby. She threw the basket full of groceries at the mongoose and killed it. Then she ran towards her baby's cradle. When she saw the dead snake near the baby's cradle, she realised the truth; the mongoose was innocent, and it had saved her baby's life by killing the snake. She also realised her mistake. She acted rashly without knowing the truth.