How is a Master Page different from an ASP.NET page?

The basic difference is that master page can't render itself (As its a layout ) but other Asp.net pages can. Master pages are something like abstract class and also it is just a template or layout. While creating web page we can specifying to web page that which template (ie Master page) need to use also we can use only one master page a time.

The MasterPage has a @Master top directive and contains ContentPlaceHolder server controls. It is quiet similar to an ASP.NET page.