The Internet Server Application Programming Interface (ISAPI) is an n-tier API of Internet Information Services (IIS), Microsoft's collection of Windows-based web server services. The most prominent application of IIS and ISAPI is Microsoft's web server.
The ISAPI has also been implemented by Apache's mod_isapi module so that server-side web applications written for Microsoft's IIS can be used with Apache. Other third-party web servers like Zeus Web Server offer ISAPI interfaces, too.
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. ISAPI is a much lower-level programming system, giving much better performance, at the expense of simplicity.