IIS overview

WWW Service
In IIS 7 and above, the WWW service no longer manages worker processes. Instead, the WWW Service is the listener adapter for the HTTP listener, HTTP.sys. As the listener adapter, the WWW Service is primarily responsible for configuring HTTP.sys, updating HTTP.sys when configuration changes, and notifying WAS when a request enters the request queue.

Additionally, the WWW Service continues to collect the counters for Web sites. Because performance counters remain part of the WWW Service, they are HTTP specific and do not apply to WAS.

Worker Processes
In IIS 7 and above, Windows Process Activation Service (WAS) manages application pool configuration and worker processes instead of the WWW Service. Each Web site runs in an application pool, which is a named worker process and logical memory allocation where one or more Web sites or applications reside. Each app pool runs in the W3wp.exe process.

Additionally, you can run WAS without the WWW Service if you do not need HTTP functionality. For example, you can manage a Web service through a WCF listener adapter, such as NetTcpActivator, without running the WWW Service if you do not need to listen for HTTP requests in HTTP.sys.

Following command can be used to list worker process

C:\Windows\System32\inetsrv>appcmd list wp

Modules
IIS include a Web server engine in which you can add or remove components, called modules, depending on your needs. Modules participates in the request processing of every request in order to change or add to it in some way.

Handler
A handler, is responsible for handling the request and producing the response for specific content types.