Typically, when we write an App for Mac OS X, we expect it to be something that is placed in the Applications
folder that the user double-clicks to launch and use. But that is not the only kind of App that we can develop.
We can also develop a plugin that rather than being installed in the Applications
folder and double-clicked, is installed in the PreferencePanes
folder and loaded by the System Preferences when the user clicks on its icon in the System Preferences.
This is commonly referred to as a plug-in because there is a special set of rules that the App must follow in order to interact with the preferences framework.
Note
While we are going to build a preference pane and install it in the System Preferences it is also possible to use preference panes in an Apps preferences and provide an Apps preference panel that is similar in style to the System Preferences.
When our preference pane is installed, it will appear in the Other section of the System Preferences...