Just about every Mac OS X application can be customized via the application's preferences settings.  Whenever a setting is altered, Mac OS X updates the application's preferences file.  These files are identified with a .plist extension.  When the application launches, OS X checks the preferences file to determine how to set the application for the user.  OS X maintains a separate preferences file for each application and each user.  For example, each user has his or her own plist file for Safari stored in the following area:

/Users/switchtoamac/Library/Preferences/com.apple.Safari.plist
/Users/getamac/Library/Preferences/com.apple.Safari.plist