This picture shows how the real servers with its real adresses and the virtual applications with its virtual addresses are shown in the network. To work with the server machines, they will have an own IP address independent from the applications. The application could be switched to this or another server machine.

The client communicates with the virtual server.

One example: A mail system is proteced by a cluster. The mail client is only able to connect the virtual address which is connected to the mail application. With this connection the client is able to work with ressources such as mailboxes, public folders, calendars and so on. It isn't important at which node this resources are located.