Web 2.0 services like Twitter can be "installed" to the GNOME desktop as an application. What this work effectively does is just create the given page as a bookmark and then when launching that application, it's spawned as a separate process from the normal Epiphany web-browser instance. The page is rendered as normal and that browser instance is bound to only the domain of that web-site. If you try to go elsewhere, such as clicking on a Twitter link, it will then launch a normal web-browser instance in another window. This application browser instance also is shown without the normal browser tool-bar or any other browser features.
That's at least how web applications are being handled now for GNOME 3.2, but it's surely to evolve in forthcoming releases. More details on this feature can be found from the developer's blog behind this work.