Error: Could not load type 'MyWebApplication.About' & Difference between CodeBehind & CodeFile

When we are trying to browse our web page in Visual Studio we got following error:
Error: Could not load type 'MyWebApplication.About'
We have a web application with the name "MyWebApplication" in which I added a new web form with the name: "About" and it generates two files "About.aspx" and "About.aspx.cs".  Now when I browse the file I got the above error. In order to resolve this error the simple trick is to change CodeBehind in the page directive to CodeFile as shown below:

Previous Page Directive is:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="About.aspx.cs" Inherits="MyWebApplication.About" %>

Now new Page Directive is:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="About.aspx.cs" Inherits="MyWebApplication.About" %>

What is the difference in CodeBehind and CodeFile ? :

 The CodeFile file About.aspx.cs file will be compiled at Runtime and saved in Temp location.
while The CodeBehind file About.aspx.cs file will be compiled at design time and embedded into the DLL.