Watch the The "Zipper" online guitar lesson by Karan Andrea from Guitar Player Wanted: Vocals A Plus
There is a terrific book by Roger Love called Set Your Voice Free, in which he describes the vocal cord movement as a zipper. This is a pretty good analogy. When you are taking a breath, your cords are completely open, and appear hinged at one end. When you are speaking or singing, the cords are "zipped" together until they are parallel.
The air passing through your cords makes them vibrate, which creates sound. When you are singing low notes, your cords are slightly further apart, and are not pulled as taut. This creates the lower pitches. As you sing higher up the scale, your cords pull closer together, although they are still parallel, and as you sing higher and higher, they stretch to create more tension for your highest notes. So this is the true action of the cords, but Roger Love's zipper concept is a bit easier to visualize.
Think of the lowest notes happening when the zipper is mostly open, and as you sing up the scale, you are zipping your vocal cords almost closed. This gives you a smooth image of your voice sliding up and down scales with no hitches. It is very helpful to visualize this zipper action when you are doing your vocal exercises, and when you are singing. Eventually, as you get more coordinated with the exercises, you will actually feel a sort of zipper action in your cords. When you get to this point, you will begin to have the control you need to sing well.