Org mode is a major mode for GNU Emacs that turns a plain-text file into a structured document for notes, planning, and publishing. The Org Manual describes it as an authoring tool and a TODO list manager for GNU Emacs, working on files with the .org extension. It was created by Carsten Dominik in 2003, and the manual’s copyright notice records the project running from 2004 onward under the Free Software Foundation. Org has since been bundled with Emacs itself.
At its core Org is an outliner. The manual explains that Org builds on Emacs Outline mode to keep large files well structured, letting the user fold and unfold sections, cycle visibility, and move whole subtrees around with a keystroke. Everything else in Org grows from this outline: a heading can carry a TODO state, a deadline, tags, and properties, so a simple bullet list becomes a project plan. Because it is all plain text, the same file is readable in any editor and survives any tool change.
From those marked-up headings Org assembles dynamic agenda views. The manual describes agendas that integrate the Emacs calendar and diary and support task-management systems such as David Allen’s Getting Things Done, pulling scheduled and deadlined items from many files into one calendar-like display. This makes Org both a writing tool and a personal information manager, which is why many users keep their entire working life in Org files.
Org is also an export engine and a literate-programming environment. The manual notes that documents can be exported to formats including HTML, LaTeX, OpenDocument, and Markdown, so a single .org source becomes a web page, a PDF, or a slide deck. Through Babel, Org supports literate programming and reproducible research: source-code blocks in many languages can be embedded in a document, evaluated, and have their results captured inline. A report and the code that produced it live in one file.
The significance of Org mode is how much it accomplishes with so little machinery. It adds no new file format beyond annotated plain text, yet it covers note-taking, project planning, calendaring, and publishing in one coherent system built entirely in Emacs Lisp. Org is among the most widely used and most copied pieces of the Emacs world, and its plain-text outline-with-metadata model has influenced a generation of note-taking and knowledge tools outside Emacs as well.