It’s really integrated into emacs.
First, you have the calendar, but it’s not just a date picker, it’s also shows holidays and other markers.
Then you have the capture (quick entry) where you have the full power of emacs environment plus lisp language to code anything you want. Emacs have other applications like a file manager, mail readers, document readers…, and you can capture the context as well as the note itself.
Then, there’s the agenda, which is fully customizable with a mix of options and code.
And there’s the exporters. Notice that emacs have support for most of the format, so it’s more like an handover to some other parts of emacs. But you don’t merely transform the document from org to html as an example. You extract the html from the org structure as you can filter sections out. Also a lot of options there (and code)
And code blocks (named babel). That you can execute.
So org can be a static document format or a dynamic environment. And all of that because of emacs as the buffer concept is very fluid. In emacs there are only buffers. Each buffer is assigned a major mode which is just a set of functions that does stuff on the buffer text. And you have the minor modes (more functions) that are more like plugins. And you’re free to hack on them. It’s just that the default set looks like a text editor.