Microsoft Exchange Server as a Development Platform
Although Microsoft Exchange Server applications can have almost any structure, they generally fall into three categories:
Client application Uses the services of a server running Microsoft Exchange Server, but resides exclusively at the user's location.
Server application Interacts with services such as the information store and the directory to provide services to users, but is not associated with any particular user.
Combined client and server application Includes a client component associated with one or more users and a server component that is a service.