A Networked Windows system is portable because of one thing:
The display functionality is separated from the application
This can be done by constructing a standard protocol,
that allows application programs to talk to the display process.
The display process (of which there is one per physical display per
machine) is called the Server, while the application programs
are Clients. In other words, the role of client and server is reversed
from that of CPU or file serving.
There can be lots of application programs. Each application program
can have 0, 1 or more Windows, which may or may not be visible
at once on the screen.
One consequence of this design is that a client can talk to a
display server on a different machine (or indeed several machines!).
The most common client program is a terminal emulation window -
application, which usually is running a command interpreter, but can
run anything else that does terminal I/O.
Figure: Servers, Clients (reverse of File case)