Mouse
Left Click: Hold to draw, release to create object
Right Click on a drawn object: Erase object
Scroll wheel: Change pen color
Left Click on the ball: Nudge ball to the right
Right Click on the ball: Nudge ball to the left

Keyboard
Space: Restart level (does not clear the object you're drawing if holding left click)

Special Drawings
In most cases drawing something makes a rigid object, but there are special objects:
Small Circle = pin (must be drawn inside an existing object, either one from the level or one you drew)
Line between two pins = rope