The Add-ons system is a WIP system similar to the mods folder. You will download add-ons for Avatar (magic , technology), and the game will automatically search for them and install them. There are also plans for it to work similary to the multi-player texturepack cache, where you will be prompted to add an add-on if the server has it added. This is planned to be release a while after the initial release, but will be quite cool .
Initial release date:
Initial idea: 3/30/13