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?

On machine B,

$ ssh -X t@C` $ echo $DISPLAY localhost:10.0` 

Does $DISPLAY shows the X server used by the X clients invoked in the ssh session? What does 10 and 0 mean?

on machine C,

$ netstat -lnt | awk ' sub(/.*:/,"",$4) && $4 >= 6000 && $4 < 6100 { print ($1 == "tcp6" ? "ip6-localhost:" : "localhost:") ($4 - 6000) }' localhost:10 ip6-localhost:10 

Does it list all the remote X servers for any X clients running on machine C? What does 10 mean?

Does 10 in both outputs mean that the X server running on machine B is listening at port number 6000+10=6010?

Thanks.