Applets are small programs that you attach and start from the GNOME's toolbar. See picture: http://www.futuredesktop.com/audio-recorder/recorder-applet.ogv
My favourite applets are:
rec-applet: Smart audio recorder applet. You can get this from Launchpad.
clipart-finder: Find and download clipart to your computer. You can get this from the Launchpad.
1) Can I use these applets in UNITY as is?
2) What changes I have to make to applets' code (rec-applet and clipart-finder) to make them usable in Unity?
3) Where is the applet-specification for Unity?