The code and content of a plug is stored in an executable binary. This binary has three basic goals:

1. Sub-class the abstract Pantheon.Switchboard.Plug class, which is a form of Gtk.Window. The contents of the plug class are "embedded" into the Switchboard application when the plug is launched.
2. Instantiate the plug class and register it with Switchboard.
3. Start the GTK+ main loop.

If you've worked with GTK+ before, you're probably thinking that this sounds very much like the workflow of a normal GTK+ standalone application. That's because it is :). The Switchboard API is designed to be easy to understand for GTK+ novices, yet still familiar to hardcore GTK+ veterans.

Generally speaking, plugs (and thus plug binaries) abstract a low-level settings backend, such as KeyFiles, gconf, dconf, or other APIs like ALSA and GStreamer, to a graphical user interface that is as adherent to the elementary HIG has possible.

The following is a very simple skeleton of a plug for changing the Desktop wallpaper. It is very well-commented, and provides a good starting point for just about every Plug.

// Sub-class the base plug from libpantheon.
public class WallpapersPlug : Pantheon.Switchboard.Plug {

    public WallpapersPlug () {

        // Add a normal GTK+ Label to the window.
        var l = new Gtk.Label("Wallpaper settings go here!");
        this.add(l);
    }
}

public static int main (string[] args) {

    Gtk.init (ref args);
    // Instantiate the plug, which handles
    // connecting to Switchboard.
    var plug = new WallpapersPlug ();
    // Connect to Switchboard and identify
    // as "Wallpapers". (For debugging)
    plug.register ("Wallpapers");
    plug.show_all ();
    // Start the GTK+ main loop.
    Gtk.main ();
    return 0;
}

Plugs are described (in a metadata sense) not by the binary, but rather by .plug files which reside in the same directory as the executable. Plug files are discussed in-depth in the next section.