Master pages are similar to ordinary ASP.NET pages.
Master pages are text files that can contain HTML, web controls, and code.
Master pages have a different file extension .master instead of .aspx.
Master pages must be used by other pages, which are known as content pages.
Master page defines the page structure and the common ingredients.
The content pages adopt this structure and just fill it with content.
You can use a Master Page to create a common page layout.
You can use Master Pages to display common content in multiple pages.
You can add multiple Master Pages to the same application.
Master page file contains a <%@ Master %> directive instead of the normal <%@ Page %> directive.
Content from the content page appears in the ContentPlaceHolder controls during merging.
You cannot cache a Master Page with the OutputCache directive.
You cannot apply a theme to a Master Page.