From https://unix.stackexchange.com/a/17278/674

If you run ssh -X localhost, you should see that $DISPLAY is (probably) localhost:10.0. Contrast with :0.0, which is the value when you're not connected over SSH. (The .0 part may be omitted; it's a screen number, but multiple screens are rarely used.) There are two forms of X displays that you're likely to ever encounter:

• Local displays, with nothing before the :.
• TCP displays, with a hostname before the :.

With ssh -X localhost, you can access the X server through both displays, but the applications will use a different method: :NUMBER accesses the server via local sockets and shared memory, whereas HOSTNAME:NUMBER accesses the server over TCP, which is slower and disables some extensions.

What are the relations and differences between X server, display and screen?

What does "the X server through both display" mean? Does a "display" means a display server, i.e. an X server, so two "displays" means two display servers, i.e. two X servers.

What does "multiple screens" mean? Does a "screen" mean a display monitor?

Thanks.