How can I make a apps, whose shape is not a rectangle or square?

[Zenith <zenithlau sniic com>]

What can I do if I want to make a small apllication, such that the such of the root window is not in 
rectangle or square in shape? For example, can I make a clock (circle shape), and there is no background 
color for the clock, or when I just start this program, ONLY, just a circle is appeared.

ps. sorry for my poor english, and I really feel hard to express this question ~~