A paragraph is a group of sentences that develops a single idea or point: 

• Structure

  A paragraph is made up of a topic sentence, body sentences, and a concluding sentence. The topic sentence introduces the main idea, and the body sentences support it. The concluding sentence can transition to the next paragraph or section. 
• Purpose

  Paragraphs help organize ideas in a logical order and guide the reader through the writing. They also show where sections of a research paper begin and end. 
Length
A paragraph is usually around 250 words and consists of five or six sentences, but this can vary. 
Transitions
Transitions are sentences that help move from one idea to the next. They can be used at the end of most paragraphs. 
Signposts
Signposts are internal aids that help readers. They can be several sentences or a paragraph that outlines what the article has covered and where it will go. 
When to start a new paragraph
Start a new paragraph when you transition to a new idea, add a new line of thought, or finish summing up.