It’s an abstract class with public abstract methods all of 
which must be implemented in the inherited classes. 

Explain him if you have ever developed an interface.

interface is reference type and contain only abstract member.
interface member can be Methods ,events, properties and indexer. but interface contain only declaration of members.
interface can't contain constant ,data field, constructor ,static members, destructors.

Interface is a MIDIOUTER between USER and MACHINE LANGUAGE

interface is a oops concept (inheritance)
its like multiple inheritance in C#.net with inheritance Key but we use Interface in VB.Net

Public Interface AnInterface //interface
  Function WhoAmI() As String
End Interface 

Public Class AClass
  Implements AnInterface 

  Public Function WhoAmI() As String Implements  AnInterface.WhoAmI
         Return "AClass"
    End Function

End Class

sorry i meant "its like multiple inheritance in C++ "

interface is an oops concept (inheritance)
its like multiple inheritance in C++ 
but we use Interface in .Net

Public Interface AnInterface //interface
  Function WhoAmI() As String
End Interface 

Public Class AClass
  Implements AnInterface 

  Public Function WhoAmI() As String Implements  AnInterface.WhoAmI
         Return "AClass"
    End Function

End Class