The Netscape Server API (NSAPI) is a direct C interface to the web server. Modules written to the NSAPI will run much faster than any other kind of dynamic content, but are capable of crashing the server. See the introduction at http://developer.netscape.com/docs/manuals/enterprise/40/nsapi/contents.htm.
Microsoft has a similar and competing API called ISAPI for its Internet Information Server. There is an introduction to ISAPI at http://www.microsoft.com/msj/0498/iis/iis.htm.