When a Mac is shared by more than one person, it’s best to set up a separate account for each user. Even if you aren’t worried about privacy, it lets everyone have their own Documents folder, Desktop, applications on the Dock, web browser bookmarks and history, and many other preferences. It also lets you only give some accounts the ability to administer the computer (for example, install applications for everyone, create accounts, make system-wide changes).
You’ll often want to switch between user accounts and be able to return to your account with all the windows just as you left them. In this video I first show you how to set up your Mac so that account switching is simple and fast. Then I show you what it looks like to switch back and forth between two accounts. It’s very handy and, of course, Apple has added their usual bit of spiffy flair when you switch.
Finally, if you want to stay logged in (with all your open windows saved) but want to lock the screen and possibly let another user log in, just choose the “Login Window…” option from the Fast User Switching menu.