What is APP Domain? What is the main role of it? Can we create custom APP Domain?

The primary purpose of the AppDomain is to isolate an application from other applications. Win32 processes provide isolation by having distinct memory address spaces. The .NET runtime enforces AppDomain isolation by keeping control over the use of memory – all memory in the AppDomain is managed by the .NET, so the runtime can ensure the AppDomain do not access each others memory.