When you’re done using your Macintosh, there are two ways to bring your session to a close: shutting down and logging out.
Because Mac OS X is a multiple-user system and a server platform, you should choose to shut down the computer only if nobody else on the network is using it or the services it provides. This includes both the other human users on the system, who might be logged into it remotely using network-shared volumes, as well as people or programs using any running network services.
For example, if you have enabled USB Printer Sharing so that others in your home or office can use the printer connected to your Mac, shutting down the Mac will also make that printer unavailable to the other users. The same applies to any web, mail, or other network services the machine may be running. On the flip side, if you are the sole account holder of your Mac, and you’re not running any public network services, then you shouldn’t encounter any problems when you shut your Mac down at the end of the day.
Logging out is the better option for Macs
that are shared by many users or that act as network servers. When
you log out (via 
→Log Out Username,
or Shift-
-Q), all programs you haven’t
quit will be quit by the system as it logs you out;
you’ll then be presented with the login screen once
again.
If other users are logged on to your Mac, either directly or through the network, they (and the programs running under them) won’t be affected by logging out of your account locally.
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