> With windows 8.1 there's now the option of moving these 2 folders to a web synced one drive folder.
Not only is this a feature in 8.1, this is a feature you can utilize in any desktop OS that OneDrive can be installed on.
> Will this allow me to sync my appdata and settings on any windows device I own?
The OneDrive tip suggests you can move your Desktop or Documents location to be under the OneDrive folder to allow syncing of the contents.
As an option, you can sync portable applications using OneDrive. This is okay because all the application files reside inside one folder.
> Does windows 8.1 by default synchronize everything in the roaming folder with my other devices? Or is this only when I'm connected to a network?
Microsoft has a detailed explanation on online-only files vs files being available offline:
http://windows.microsoft.com/en-us/windows-8/onedrive-online-available-offline