Applets are GNOME applications which reside in the Panel. An applet's appearance typically reveals the state of the applet or other information. Applets often have buttons, sliders, entries, or other methods to allow you to control their behavior.
Some example applets are shown above. At the very left is the Sound Monitor Applet, which displays the current volume of sound being played and allows you to control various sound features. Second is the Mixer Applet, which allows you to turn on/off sound and control its volume by clicking on applet. This is followed by the Battery Power Applet, useful for monitoring the battery on a laptop. The fourth applet is the GTCD Applet, a CD player which has all its controls available in the applet and displays the track and time. The next applet is the Drive Mount Applet, used to mount and unmount drives with a single click of the mouse. The last applet shown is an analog clock, displaying the current time.
GNOME has many useful applets. Just right click on your Panel and add some applets to your panel to find out what is available and which ones are best for you.