Heck, for that matter I suppose you could use the unmanaged Windows API  
via p/invoke in C# if you really wanted to.

To elaborate on Morten's reply, generally when you see someone talking  
about "WinForms" or some variant of the term, they are referring to the  
classes found in .NET in the System.Windows.Forms namespace.

In the context of C#, I would not say that there _is_ a clear difference  
between "winforms" and "Windows based application development".  The  
latter is, to me, a broader term that includes "winforms" and any other  
method for writing GUI applications on Windows.  Alternatively, I would  
say that even with the existence of WPF, if you say that you're writing a  
Windows application in C# and don't provide any further qualification, the  
usual assumption is that you're using the Forms namespace (though  
obviously that assumption could be incorrect).

Pete