Wikipedia
Org-mode (also: Org mode; ) is an editing and organizing mode for notes, planning, and authoring in the free software text editor Emacs. The name is used to encompass plain text files ("org files") that include simple marks to indicate levels of a hierarchy (which could be the outline of an essay, a topic list with subtopics, nested computer code, ...), and an editor with functions that can read the markup and manipulate hierarchy elements (expand/hide elements, move blocks of elements, check off to-do list items, ...).
Org-mode was created by Carsten Dominik in 2003, originally to organize his life and work, and since the first release numerous users and developers have contributed to this free software package, Emacs includes Org-mode as a major mode. Bastien Guerry is the current maintainer, in cooperation with an active development community. Since its success in EMACS, some other systems have also begun providing functions to work with org files.