A paragraph is a self-contained unit of a discourse in writing dealing with a particular point or idea. Paragraphs consist of one or more sentences. The start of a paragraph is indicated by beginning on a new line. Sometimes the first line is indented. At various times, the beginning of a paragraph has been indicated by the pilcrow: ¶.
A written work — be it an essay or a story — is about an idea or concept. An essay explains it; a story narrates it. To help the reader understand and enjoy it, the explanation or narration is broken down into units of text, the paragraph. In an essay, each paragraph explains or demonstrates a key point or thought of the central idea, usually to inform or persuade. In fiction, each paragraph serves to advance the plot, develop a character, describe a scene or narrate an action — all to entertain the reader. All paragraphs support each other, leading the reader from the first idea to the final resolution of the written work.
Some styles do not indent the first paragraph, but do indent all those that subsequently follow. This follows the logic that the purpose of indenting is to separate paragraphs in a way that lets the reader know where one paragraph finishes and another begins. The general American practice is to indicate all paragraphs including the first, by indenting the first line (three to five spaces), whereas business letters generally use blank lines and no indent (these are sometimes known as "block paragraphs"). For other purposes, indented paragraphs are preferred. Most published books use a device to separate certain paragraphs further when there is a change of scene or time. This extra space, especially when co-occurring at a page break, may contain an asterisk, three asterisks, a special stylistic dingbat, or a special symbol known as an asterism.
In literature, a "detail" is a small piece of information within a paragraph. A detail usually exists to support or explain a main idea. In the following excerpt from Dr. Samuel Johnson's Lives of the English Poets, the first sentence is the main idea: that Joseph Addison is a skilled "describer of life and manners". The succeeding sentences are details that support and explain the main idea in a specific way:
As a describer of life and manners, he must be allowed to stand perhaps the first of the first rank. His humour, which, as Steele observes, is peculiar to himself, is so happily diffused as to give the grace of novelty to domestic scenes and daily occurrences. He never "o'ersteps the modesty of nature," nor raises merriment or wonder by the violation of truth. His figures neither divert by distortion nor amaze by aggravation. He copies life with so much fidelity that he can be hardly said to invent; yet his exhibitions have an air so much original, that it is difficult to suppose them not merely the product of imagination.