The Mac OS X Server v10.5 User Management Guide describes the behavior of a Guest Computer as:  If an unknown computer (one that doesn’t have a computer account) connects to your network and attempts to access services, that computer is treated as a guest computer. Settings for the guest computer account apply to these unknown computers.  (page 107)

Read more here:
Mac OS X Server v10.5 User Management Guide: Guest Computer behavior clarification

Similar Posts:

• Mac OS X Server v10.5 and later: Mobile users may not be able to log in when using a password policy
• Mac OS X Server v10.5 and later: Mobile users may not be able to log in when using a password policy
• Back to My Mac sometimes connects as Guest
• Mac OS X Server: Don’t log in to the server with a network user’s account
• Mac 101: File sharing