Microsoft's web server application software is called Internet Information Services, which is made up of a number of "sub-applications" and is very configurable.
ASP.NET is one such slice of IIS, allowing a programmer to write web applications in their choice of programming language (VB.NET, C#, F#) that's supported by the Microsoft .NET CLR.
Both Filters and Extensions must be compiled into DLL files which are then registered with IIS to be run on the web server.
ISAPI applications can be written using any language which allows the export of standard C functions, for instance C, C++, Delphi.
Additionally, there is the ATL Server technology which includes a C++ library dedicated to developing ISAPI applications.