What is an application domain?

Previously "PROCESS" were used as the security boundaries. One of the process has its own virtual memory and does not overlap the  another process virtual memory; due to this one process cannot crash the another process. Therefore any problem or error in one process does not affect  the process. In .NET they want one step ahead introducing application domains. In the application domains multiple applications can run in same process without influencing each other. If one of the application domains gives error it does not affect the other application domains. To invoke method in a object running on different application domain .NET remoting is used.