* '''Built-in:''' Applications and functionality that come built-in in the HDA base system. They cannot really work outside.

* '''HDA plug-ins:''' These are applications that are specific to the HDA. They are mostly implemented on [http://rubyonrails.com Ruby on Rails], though they can be implemented in PHP, Python, Flash, or any other language, like C, C++, etc. We have a few of these applications at the moment.

* '''HDAized applications:''' these are applications that, with little modification, have been adapted to run on the HDA seamlessly or nearly seamlessly. We have webcal (which is an HDAized version of [http://phpicalendar.net/ PHP iCalendar]).