IBM HTTP Server is a full-featured web server included with WebSphere Application Server. It is based on Apache HTTP Server (httpd.apache.org).
Web server plug-ins enable the web server to communicate requests for dynamic content, such as servlets, to the application server. A web server plug-in is associated with each web server definition. The configuration file (plugin-cfg.xml) that is generated for each plug-in is based on the applications that are routed through the associated web server.A web server plug-in is used to forward HTTP requests from a supported web server to an application server.
To learn more, here are some helpful WSTEs for IBM HTTP Server and the WebSphere Plugin: