In the 3-D business, there are two major categories of 3-D glasses: passive and active. Passive lenses rely on simple technology and are probably what you think of when you hear the term 3-D glasses. The classic 3-D glasses have anaglyph lenses.
Anaglyph glasses use two different color lenses to filter the images you look at on the television screen. The two most common colors used are red and blue. If you were to look at the screen without your glasses, you would see that there are two sets of images slightly offset from one another. One will have a blue tint to it and the other will have a reddish hue. If you put on your glasses, you should see a single image that appears to have depth to it.
What's happening here? The red lens absorbs all the red light coming from your television, canceling out the red-hued images. The blue lens does the same for the blue images. The eye behind the red lens will only see the blue images while the eye behind the blue lens sees the red ones. Because each eye can only see one set of images, your brain interprets this to mean that both eyes are looking at the same object. But your eyes are converging on a point that's different from the focal point -- the focus will always be your television screen. That's what creates the illusion of depth.
Today, a more popular type of passive lenses in movie theaters can be found in the polarized glasses. Again, if you look at a screen that uses this technology you'll see more than one set of images. The glasses use lenses that filter out light waves projected at certain angles. Each lens only allows light through that is polarized in a compatible way. Because of this, each eye will see only one set of images on the screen. Polarized lenses are becoming more popular than anaglyph glasses because the glasses don't distort the color of the image as much and provide a better audience experience. But it's very difficult to use the polarization technique for home theater systems -- most methods would require you to coat your television screen with a special polarizing film first.
Now let's take a look at active glasses.