a) By extending ContainerControl, UserControl inherits all the standard
positioning and mnemonic-handling code that is necessary in a user
control.The UserControl gives you the ability to create controls that
can be used in multiple places within an application or organization.
a usercontrol from winform renders it's apparentness by GDI or GDI+.
System.Object
System.MarshalByRefObject
System.ComponentModel.Component
System.Windows.Forms.Control
System.Windows.Forms.ScrollableControl
System.Windows.Forms.ContainerControl
System.Windows.Forms.UserControl
b) A usercontrol from webforms, it represents an .ascx file, also known
as a user control, requested from a server that hosts an ASP.NET Web
application. The file must be called from a Web Forms page or a parser
error will occur.
a usercontrol from webform renders it's apparentness by Html elements
based on browser
System.Object
System.Web.UI.Control
System.Web.UI.TemplateControl
System.Web.UI.UserControl
Reference from MSDN.