Page views are one of the pillars of the paradigm of separation between logic, data and presentation. They are solely responsible for the presentation part. No other classes are allowed to output HTML (except for debugging purposes). Each page (WP file) has at least one view defined in its init() method.
All page views are inherited from Web_Page_View class.
By default Tangra framework uses » Smarty template engine. It is wrapped in tple_smarty module. Theoretically you can use other template engine if you create wrapper around it (i.e. class that implements I_Tple) but since Smarty is the leading and most powerful template engine - it does not make much sense.
All presentation HTML code is contained in so called Tpl files. Those files are used by the template engine to produce final HTML output. Simplified it works like this:
init()run() or event handling method you "export" variables with $this->export('somevarname', $somevar)run() or event handling method returns viewshow() methodTple (which is wrapper around Smarty tple), passes all exported variables to it.