http://developer.ubuntu.com/web/tutorial/ . It's Unity-centric, but has a lot of good information.
There is not really a "Unity" webapp. Webapps are applications. Good applications work with many desktop environments.
In Xubuntu, applications (including webapps) use the Applications menu, have indicators and controls as a Panel Plugin, and the user can add them to a toolbar shortcut if they wish.
In Unity, applications (including webapps) show up in Dash, have indicators and controls in the appropriate AppIndicator on the top bar, and the user can add them to the Launcher Bar is they wish.