Asp.Net introduces the concept of an Application Domain which is shortly known as AppDomain. It can be considered as a Lightweight process which is both a container and boundary. The .NET runtime uses an AppDomain as a container for code and data, just like the operating system uses a process as a container for code and data. As the operating system uses a process to isolate misbehaving code, the .NET runtime uses an AppDomain to isolate code inside of a secure boundary.

The CLR can allow the multiple .Net applications to be run in a single AppDomain.

The CLR isolates each application domain from all other application domains and prevents the configuration, security, or stability of a running .NET applications from affecting other applications.An AppDomain can be destroyed without effecting the other Appdomains in the process.
 
Mulitple Appdomains can exist in Win32 process. As we discussed the main aim of AppDomain is to isolate applications from each other and the process is same as the working of operating system process. This isolation 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 address space.

Win32 processes provide isolation by having distinct memory addresses. The .Net runtime enforces AppDomain isolation by keeping control over the use of memory. All memory in the App domain is managed by the run time so the runtime can ensure that AppDomains Do not access each others memory.