Simple: Apple menu, Restart… or Shut Down… commands, as appropriate. But what do you do when your Mac has frozen and you cannot use the menu commands, or you try to use them and nothing seems to happen?

As I write in many places, the last thing that you should feel forced to do is to turn off the mains power, or to pull the power cable. For a start, that won’t work with a laptop anyway, and it is very likely to leave your Mac in a mess. Shutting down, or restarting, is an orderly process which has several important tasks to complete. If you don’t give your Mac the chance to complete them, or even start them, any storage is at risk of suffering (soft) damage, you may lose work or data, and exceptionally could even damage hardware.

If you have had to do this, or the mains power went off and you don’t have a UPS to allow orderly shutdown, it is often wise to start up in Recovery mode and run Disk Utility’s First Aid. Yes, OS X should ‘replay the journal‘ to do its best to commit any changes it needs to your startup volume, but that can still leave problems to trip you up later.

If you cannot use the mouse/trackpad pointer to operate the Apple menu, then these are the available shortcuts which should ensure shutdown or a restart, or log you out:

• To force shut-down – press and hold the Power button for 5 seconds, or until it shuts down.
• To close all apps and then force shutdown – Control-Option-Command-Power button, or Control-Option-Command-Eject.
• To force restart – Control-Command-Power button.
• To close all open apps and then force restart – Control-Command-Eject.
• To log out normally – Shift-Command-Q.
• To log out immediately – Option-Shift-Command-Q.

Pressing and holding the Power button is the most widely-used and reliable alternative. If your Mac has simply frozen, with the clock stopped, mouse/trackpad and keyboard both dead, then it is often worth giving it a minute or two to restart automatically, before reaching for the Power button. That is likely to have been a new-style kernel panic, which should now be handled by an automatic restart.

If the screen goes all black and you see the ‘busy’ indicator, that means that shutdown is in progress. Sometimes that can take a good while before it completes: don’t get impatient, give it several minutes before wondering whether it will end in shutdown.

If your Mac is on a local network, you have enabled Remote Login in the Sharing pane, set up and tested your account in Terminal’s ssh (see man ssh for details), you can connect from another Mac and run the command
shutdown -r now to restart, or
shutdown now to shut down.
Because this operates at a lower level than the GUI, it can succeed where other attempts have failed. This is particularly useful for anyone acting as a sysadmin on a local network. Pressing the Power button is also low-level, working with the SMC, but can rarely play up.

If you have tried all these, and your Mac is still unresponsive, try the Power button a few more times, each time holding for a good ten seconds, and giving it several minutes between attempts. Only then should you consider turning the mains power off if it is a desktop Mac, or removing the battery with the mains power adaptor disconnected if a laptop. But those really are last-ditch measures.