How to Log Off of or Use Fast User Switching on Your Mac
If you’re ready to call it quits for the day but don’t want to shut down your Mac, but you don’t want to leave your account open for anyone with prying eyes, you’re in luck. The way to shut down without really shutting down is to choose the Apple key→Log Out.
Or say that you're in the middle of working when you need to take a break. As you get up to leave, your spouse comes running in, Honey, can I quickly check my e-mail? You could log out to let her do so, but because you’re going to be right back, you should take advantage of the fast user switching feature. To use fast user switching, you must have previously checked the box next to Enable Fast User Switching, in the Login Options window.
Then, to let any other user use the computer while you take a quick break, click your user name in the upper-right corner of the screen. A list of all account holders appears. The person can then click his name and type a password. Like a revolving door, your entire desktop spins out of the way while the other user’s desktop spins in. When you return moments later, you repeat this procedure by choosing your name and password. Your desktop twirls back into view, right where you left off.