Master page allows developer to share the same content or create common layout among multiple pages in ASP.NET. For example you can share the same header, footer, sidebar and navigation among all your pages so that they have the same look and feel
It is better to use CSS with Master pages. Once your master page has been created then it is easy to maintain your codes. If you are not satisfied with a shared content, such as header or footer, all you have to do is modify the Master page and the modification would be reflected across all pages that you have applied the Master page to.
Whenever a master page is created, it uses the Master page directive and not the normal Page directive. Also Master pages use the ContentPlaceHolder to mark the area which can be modified in the page content that inherit from the Master page
By default all pages will inherit the Title of the Master page unless you specify a value for the Title in each page. The Titles of pages can be modified by using the Title attribute in Page directive.