Answer:  When you start a computer which has more than one user, the computer usually stops at something called the "log-in screen."  On this screen, the various users will be listed and whoever wants to use the computer clicks on his or her name and signs in.  We would then say the user is "logged in."
 
When another person wants to use the computer, the question comes up: should the first user "log off" or should they click on "switch user?"
 
Let's look at the difference.  When a user selects "log off" all of their programs and files are closed down - as if the computer was being turned off - and you are returned to the log-in screen.  On the other hand, when "switch user" is selected, all of the programs and files in use by the first user stay open even though you are also returned to the log-in screen.  Now, when the second user clicks on his or her name and logs in, all of his or her programs open as well.
 
Thinking about how the "switch user" option works, this means that the computer's memory and the CPU are being used by both accounts.  This will often result in the computer slowing down tremendously and, perhaps, even freezing up.

So, what's the bottom line answer?  If the first user is finished with the computer for the time being, they should LOG OFF.  If the second user just wants to hop into his or her account to grab a file or send an email and is only going to be logged in for a couple of minutes, then they should use SWITCH USER.  After they've finished, they should LOG OFF.