Markdown is a lightweight markup language created by John Gruber in 2004. Its core design principle is that the plain text should be as readable as possible before any rendering takes place. Simple punctuation characters denote formatting: hash characters for headings, asterisks for bold and italic, hyphens for bullet lists, numbers for ordered lists, backticks for inline code, and brackets with parentheses for links.

Markdown has evolved into several dialects. CommonMark is a rigorously specified standard. GitHub Flavored Markdown (GFM) adds tables, task lists, and syntax highlighting. Most platforms support a variant of GFM or CommonMark with minor variations.

Markdown has become the de facto standard for developer documentation, technical writing, and content creation across the web. GitHub renders Markdown in README files, issues, pull request descriptions, comments, and wikis. Static site generators like Jekyll, Hugo, Gatsby, and Eleventy use Markdown files as the source for blog posts and pages.

Note-taking applications including Obsidian, Notion, Bear, and Typora use Markdown as their native format, and many developers write all their notes in Markdown to keep them portable. A Markdown previewer lets you see exactly how your content will render before you publish or commit it, helping you catch broken links, improperly formatted code blocks, and heading hierarchy errors before they appear live.