Use a site to define a collection of settings, such as horizontal and vertical navigation, theme, header form and default form that determine the makeup of a solution.

See Create a site.

The same form can be used in multiple sites, but the look and feel changes based on the theme, header and menus associated with the site.

You can specify Header Form, Horizontal and Vertical Navigation, Default Form, Theme, and Languages to be used for your Desktop, Tablet, and Phone forms.

At runtime, the form is displayed depending on the device from where it is viewed. For example, if the form is being viewed from a desktop, the form will be displayed based on the desktop settings (unless a specific Header Form, Horizontal Navigation, Vertical Navigation has been selected in the form). This means that you could have different layouts for the same form depending on the settings, and the device on which you are viewing the form.