Application pool in IIS
It defines a group of one or more worker processes, configured with common settings that serve requests applications that are assigned to that application pool. It allow a set of Web applications to share one or more similarly configured worker processes and provide a convenient way to isolate a set of Web applications from other set of Web applications on the server computer. In it there is are process boundaries which separate each worker process and because of this reason application problems in one application pool do not affect Web sites or applications in other application pools.
