In this animation, you can see D-glucose switch between its different forms (or isomers):
α-glucose to the open chain form to β-glucose, back to the open chain form, and then back to α-glucose (and on and on and on...)

To help make this easier to see, we've used a short-cut way of drawing molecules that doesn't show some of the carbons and hydrogens. Click here for more info about drawing molecules like this.

When the ring opens up, watch the red O and the blue H rotate around. So, when the ring closes again, that red O can be either out or down.

In the α-glucose, that red O is pointed down. In β-glucose, it's pointed out.

Watch the green carbon - in each structure it always has four bonds.

And Even More Info....

If D-glucose molecules are dissolved in water, some molecules will be in each one of these forms. Most of them will be in the β-glucose form, though, because that's the most stable. It turns out that in a ring like this, when all the -OH groups are sticking out, the molecule is the most comfortable, that is, it has the most amount of "elbow room", or, the least amount of "clutter", so to speak. (O.k., do ya want the big cheesy science talk?! Here it is! β-D-glucose has less 1,3-diaxial interactions and less steric hindrance than α-D-glucose. Phew!)

Note: (O.k, now this is a disclaimer to all the folks who want to know exactly how the ring opens and closes...) There's more to the mechanism of ring opening and closing that's really not shown here; this animation is intended to illustrate what happens as opposed to exactly how it happens.