Application Pool is one of the new and very good feature in IIS 6. Application pools are used in isolating Web applications from one another and thereby increasing the reliability of your ASP.NET Web applications. Application pools allow us to make the asp.net web applications robust.
An Application Pool contains one or more web applications and allows us to configure a level of isolation between different Web applications. If we want to isolate all the Web applications running in the same computer, we can do this by creating a separate application pool for each Web application and keep them in their corresponding application pool. Because each application pool runs in its own worker process, errors in one application pool will not affect the applications running in other application pools. Deploying applications in application pools is a primary advantage of running IIS 6.0 in worker process isolation mode because you can customize the application pools to achieve the degree of application isolation that you need.