Modifies the defaults system. When you customize your Mac using the System Preferences, all those changes and settings are stored in the defaults system. Everything that you've done to make your Mac your own is stored as XML data in the form of a property list (or plist). This property list is, in turn, stored in ~/Library/Preferences.
Every time you change one of those settings, that particular property list is updated. There are two other ways to alter the property lists: using the Property List Editor application (/Developer/Applications) or using the defaults command in the Terminal. Whether you use System Preferences, Property List Editor, or the defaults command, any changes you make affect the current user.
Perform operations on the local machine.
Perform operations on the specified host.
Print all your current settings.
Print your settings for the specified domain, such as com.apple.dock.
Print the value of the specified key. For example, to see the current Dock orientation, use: defaults read com.apple.dock orientation
Print the datatype of the specified key. For example, defaults read-type com.apple.dock orientation tells you that the type of the orientation key is
domain key value
Write a value to the specified key.
domain old_key new_key
Rename the specified key.
Delete the specified ...