Well, WinForms is a framework for creating windows based applications in
.Net. This is not the only one as with .Net Framework 3.0 you can use
WPF
(Windows Presentation Foundation) instead.
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