A dynamic-link library (DLL) that runs in the same address space as the Web server running Internet Information Services (IIS) and can access all available resources on the Web server.
ISAPI extensions (sometimes called ISAPI Server extensions) can run in process or out of process on IIS. These extensions are generally loaded on demand the first time a user requests them and remain in memory until the service is stopped.
You can use ISAPI extensions wherever you might use Common Gateway Interface (CGI) applications, such as for a form handler for Hypertext Markup Language (HTML) forms. An ISAPI extension is generally called in a manner similar to calling a CGI application.
For example, the Uniform Resource Locator (URL) shown below invokes the extension TEST.DLL on the Web site www.northwind.microsoft.com and passes it the parameters Value1 and Value2: