For a description of what a page is see blog post.
Why would you want to place a page within the browser?
It means you can run the WPF application over the internet or an intranet without any installation at all.
Thus far,I’ve mainly seen browser applications used as demos.
You create one using the visual studio new wpf browser application template.
What is useful though, is that you can write all your views (or dialogs) as pages by using the <page> xml construct.
Then you can put them into a class library (dll to us old-uns).
That enables you to have one project that is a WPF application, and one browser application both references the common class library.
Both types of application can reference the pages, and bingo you maintain your ui logic in one place.