Single user mode is a method of booting the Mac OS into a text only interface in which your are the BSD "root" user.  To boot the computer into single user mode hold down the apple key and the s key after restarting.  The root user in a UNIX-like OS can do anything to the system without concern for file and folder permissions.  This is good and bad as root can repair many problems but can also delete critical files without warning.  When booting into single user mode be careful to type commands exactly as given to avoid mistakes.  One of the issues that may occur in Mac OS X is that a computer may boot fine but never show the Finder only the default or custom background picture for the auto logged in user.  This issue may be as a result of a bad preferences file named .GlobalPreferences.plist (note the period at the beginning of the name).  There are actually up to 3 .GlobalPreferences.plist files in different places: /Library/Preferences, ~/Library/Preferences, and ~/Library/Preferences/ByHost folders.  The no Finder issue can be fixed by removing each of these files with rm in single user mode.