App domain or application domain in ASP.NET is a light-weight process that has its own set of code, configuration and data settings. It is a logical boundary that separates one application from accessing or interfering with others. App domains help in better utilization of resources by using fewer processes for executing different applications. For example, ASP.NET is a runtime host that creates different application domains for every user who accesses a web site. These can be created and set up for apps that need to isolate code or to load extensions dynamically.