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.
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.