You plug an external monitor or projector into your Macbook Air or Pro, and what do you see? It's anyone's guess. It could be an extension of your desktop or a reproduction of what's on your laptop. And it might not be what you want to see on the big screen. Don't mess around with System Preferences. Learn this keyboard shortcut:
COMMAND + F1
Holds down the Apple command key and press that chiclet F1 key above the 1 and next to Esc.
This toggles between display modes: If you were mirroring, it'll extend your desktop. If your desktop was extended, it'll mirror.
This is a handy trick to have up your sleeve when you're presenting to a group and you don't want them to watch you cue up what you'll show them next. Use extended desktops to do work behind the scenes and hit Command+F1 for the big reveal. Then Command+F1 again to hide your work.