What happens when I press the Power button and what are the power states?

When the computer is off, press the Power button to turn it on. When the computer is on, press it briefly to initiate Sleep. From Sleep or Hibernation, press it briefly to exit those states. Pressing and holding the power button results in loss of unsaved information, and holding it for at least 5 seconds will turn off the computer if it has stopped responding. (Page 11)