Before Giving the Definition: you can say like this, Concept of Application pool has from IIS 6.0.

Application pools are used to separate sets of IIS worker processes that share the same configuration and application boundaries. Application pools used to isolate our web application for better security, reliability, and availability and performance and keep running with out impacting each other . The worker process serves as the process boundary that separates each application pool so that when one worker process or application is having an issue or recycles, other applications or worker processes are not affected.

One Application Pool can have multiple worker process Also.

Main Point to Remember:

★ Isolation of Different Web Application

★ Individual worker process for different web application

★ More reliably web application

★ Better Performance