Why code behind in a user control is an abstract class?

[José]

for example in Portal Starter Kit there is an Announcements user control and the "controller?" class begins wit

Public MustInherit Class Announcement

Why MustInherit

What does Inherits mean At Control and Page directive
<%@ Control language="vb" Inherits="ASPNET.StarterKit.Portal.Announcements" CodeBehind="Announcements.ascx.vb" AutoEventWireup="false" %

Thank yo