what is the use of appdomain?

Operating systems and runtime environments typically provide some form of isolation between applications. For example, Microsoft Windows uses processes to isolate applications. This isolation is necessary to ensure that code running in one application cannot adversely affect other, unrelated applications.

Application domains provide an isolation boundary for security, reliability, and versioning, and for unloading assemblies. Application domains are typically created by runtime hosts, which are responsible for bootstrapping the common language runtime before an application is run.

What is application domain ?

The application Domain is used to isolate an application from other applications. One process has its own virtual memory and does not over lap the other process's virtual memory; due to this one process can not crash the other process. As a result any problem or error in one process does not affect the other process. In .Net they went one step ahead introducing application domains. In application domain multiple applications can run in same process with out influencing each other. If one of the application domains throws error it does not affect the other application domains.