Learn how to set up guest accounts on your Mac. Guest accounts are a great way to allow friends or your kids to use your Mac without worrying about them messing with your documents or settings. Guest accounts are created on the fly and deleted after use. You can set parental controls for guest accounts and also save documents out of them using the Shared folder.
If more than one person uses your Mac, you should set up multiple user accounts so each person has their own space for files and their own application preferences. You can use Fast User Switching to keep multiple accounts logged in at the same time so applications remain running and documents remain open.