System.Collections.IEnumerator

Added in .NET 1.1
supports a simple iteration over a non-generic collection
This allows you to implement a non-standard way of enumerating something, ie returning its elements one-by-one

public interface IEnumerator 
{ 
   object Current { get; } 
   bool MoveNext(); 
   void Reset(); 
} 

Current - (Property) Throws an exception if MoveNext returns false
MoveNext - (Method) sets Current to the next element. If MoveNext passes the end of the collection, the enumerator is positioned after the last element and MoveNext returns false
Reset - (Method) Sets the enumerator before the first element

Collections do not usually implement enumerators
Collections usually provide enumerators via the IEnumerable interface

System.Collections.IEnumerator<T>

Added in .NET 2.0
This inherits from IDisposable

public interface IEnumerator<T> : IEnumerator, IDisposable 
{ 
   T Current { get; } 
}