First of all, you should know the usage of ascx page?
The main usage of a ascx file: It is a
user control, is a kind of composite control that works much like an ASP.NET Web page.
To a user contol:
*You can add existing Web server controls and markup to a user control
*Define properties and methods for the control.
*You can then embed them in ASP.NET Web pages.
How to use it in your aspx pages:
Call it in your page's head section.
<![CDATA[<%@ Register Src="~/folder_name/YourControl.ascx" TagName="YourControl" TagPrefix="urCtrl" %>]]>
The functionality of a master page is totally different.
Good luck,
OI