It is easy to start, but how do you turn the darn PC off?

I wish this was an easy question but the truth is it depends. What it depends on is your operating system. There are 3 major categories of operating systems for personal computers Apple (OS-X),Windows, Linux. Read the comments that this generates and you will see how complicated this topic is, or you can go to Wikipedia Operating Systems.

So for

• Windows You can keep hitting  Alt-F4 That is hold down the Alt key like it is a shift key and tap F4 on the top row of your keyboard.
• OS-X Command + Option + Control + Eject (or Power Button). I am taking the word of Google because I am a Windows user.
• Linux you just tap the power button once.