Application Domain

An application domain is the CLR equivalent of an operation system’s process . An application domain is used to isolate applications from oneanother. This is the same way an operating system process works. The
separation is required so that applications do not affect one another.This separation is achieved by making sure than any given unique virtual address space runs exactly one application and scopes the resources for the process or application domain using that addess space.However, a CLR application domain is seperate from a process. It is contained within an operating system process. A single CLR operating system process can contain multiple application domains. There are some major pluses to having application domains within a single process.

• Lower system cost – many application domains can be contained within a single sytem process.
• The application in an application domain can be stopped without affecting the state of another application domain running in the same process.
• A fault or exception in on application domain will not affect other application domains or crash the entire process that hosts the application domains.
• Configuration information is part of an application domains scope, not the scope of the process.
• Each application domain can have different security access levels assigned to them, all within a single process.
• Code in one application domain cannot directly access code in another application domain. (* see below)

So you see, the CLR is like a mini-operating system. It runs a single process that contains a bunch of sub-process, or application domains.
* Direct communication cannot be acheived across application domains. Application domains can still talk to eachother by passing objects via marshalling by value (unbound objects), marshalling by reference through a proxy (AppDomain-bound objects). There is a third type of object called a context-bound object which can be marshalled by reference across domains and also within the context of its own application domain. Because of the verifiable type-safety of managed  code, the CLR can provide fault isolation between domains at a much lower cost than an operating system process can. The static type verification used for isolation does not require the same process switches or hardware ring transitions that an operating system process requires.