If you look at any essay, you will see that it is divided up into sections, the first line of each being indented slightly to the right. These sections are called Paragraphs. Essays are broken up into paragraphs to make the reading of them easier.
Definition -- A paragraph is a number of sentences grouped together and relating to one topic; or, a group of related sentences that develop a single point.
There is no rule as to the length of paragraphs. They maybe short or long according to the necessity of the case. A paragraph may consist of a single sentence, or of many sentences.