Application Pools & Services in IIS 6

When you run IIS 6.0 in worker process isolation mode, you can separate different Web applications and Web sites into groups known as application pools. An application pool is a group of one or more URLs that are served by a worker process or set of worker processes. Any Web directory or virtual directory can be assigned to an application pool.

Every application within an application pool shares the same worker process. Because each worker process operates as a separate instance of the worker process executable, W3wp.exe, the worker process that services one application pool is separated from the worker process that services another. Each separate worker process provides a process boundary so that when an application is assigned to one application pool, problems in other application pools do not affect the application. This ensures that if a worker process fails, it does not affect the applications running in other application pools.
Reference

IIS 6.0 Services:
IIS 6.0 offers four Internet services (WWW Service, FTP, SMTP, NNTP) that you can use to create sites or virtual servers, configure properties and security settings, and set up components to customize your system.

When you install IIS 6.0 on a computer that does not contain an earlier version of IIS, IIS 6.0 automatically installs the following two services:
1. The WWW service, which hosts Internet and intranet content
2. The IIS Admin service, which manages the IIS Metabase

You can also choose to install one or more of the following services:
1. The FTP service for hosting sites from which users can upload and download files
2. The NNTP service for hosting discussion groups
3. The SMTP service for sending and receiving e-mail messages

Reference: IIS 6 Services

World Wide Web Publishing Service:
The World Wide Web Publishing Service (WWW service) provides Web publishing for IIS, connecting client HTTP requests to Web sites running on an IIS-based Web server. The WWW service manages and configures the IIS core components that process HTTP requests. These core components include the HTTP protocol stack (HTTP.sys) and the worker processes.

IIS Admin Service:
IIS Admin service is a Windows Server 2003 service that manages the IIS metabase. The metabase stores IIS configuration data in a plaintext XML file that you can read and edit by using common text editors. IIS Admin service makes metabase data available to other applications, including the core components of IIS, applications built on IIS, and applications that are independent of IIS, such as management or monitoring tools.