If you experiment with this, it'll be clear:
/etc/rc2.d, you'll find files that are instructions what your computer should do when it starts.
If you use GNOME, look for a file with
gdm in its name, then replace the
S (first letter of the name) by a lowercase
s. (GDM is as you might have guessed the GNOME display manager. If you use some other suite, of course, find out what display manager it uses, then disable it in the same way.)
Now, reboot your computer and you'll notice that the login screen no longer shows up. Conclusion 1: No display manager, no login screen.
Instead, you'll log in to the console. Now, try for example to play a movie (or do anything with graphics). Won't work! This is because X isn't running. Conclusion 2: No X, no graphics.
Third step, starting X: By typing
xinit, depending on your
~/.xinitrc file, a set of applications might get started. But, to illustrate, try running X with only a terminal, say, urxvt. So, put
.xinitrc and comment-out (with
#) everything else, then type
You should now see the
urxvt window. Here, you could play movies, etc. But instead, let's move the
urxvt window somewhere else. Can't do it. Conclusion 3: No window manager, none of the usual GUI functionality you are probably used to. So, type
urxvt. (That command will exit the terminal, but, as that was the only process run as specified in
.xinitrc, X will terminate as well.)
Last step, modify
.xinitrc once more:
& so the processes will run concurrently)
Run X again and see the result. The last part of the puzzle: metacity, a window manager.
To get out, type
pkill -9 metacity. (On this, urxvt will terminate as well, possibly because, as it was run in the background (with
&), metacity is the only process X monitors.)
Good luck. Probably, you'll get stuck on some detail, but it's worth it, to gain understanding.