One difference is that web forms start with the Page directive, a master page starts with a Master directive that specifies the same information, as shown here:

<%@ Master Language="C#" AutoEventWireup="true" CodeFile="SiteTemplate.master.cs" Inherits="SiteTemplate" %>

Another difference between master pages and ordinary web forms is that master pages can use the ContentPlaceHolder control, which isn’t allowed in ordinary pages. The ContentPlaceHolder is a portion of the page where the content page can insert content.