It is an extension of Emacs outline mode. Outliners are hierarchical text formats. Org added many useful things to outline mode: keywords, tags, timestamps, foo

It is an extension of Emacs outline mode. Outliners are hierarchical text formats. Org added many useful things to outline mode: keywords, tags, timestamps, footnotes, links, tables... What makes Org unique is the presence of interpreters, including user-defined ones, that take Org files as input and do things on them. A famous interpreter is org-agenda which harvests TODO keywords, timestamp deadlines, etc. from different Org files to generate a weekly agenda. Another one is Babel which runs embedded code in Org files, i.e. a literate programming system. To sum up, Org is a plain text format analogous to Markdown, but with many more features and also interpreters that define certain operations on them.