How do I directly boot to a terminal, bypassing X11?

Question

I am seeking a help regarding the following issues:

1. How to turn off the X server when I have already logged into Ubuntu
   (I know about startx to start the X server but don't know how to shut it down).

2. When I start my machine, Ubuntu directly boots automatically X. What I need to do if I want to boot to plane terminal instead of X?
   (after that I can use startx to start the X if I feel so).