There are several ways to make a film 3D: anaglyph images, polarization systems, what's known as the "eclipse method," the "Pulfrich effect," interference filter technology, spectral separation, lenticular printing, parallax barrier technology, and autostereoscopy

Stereoscopy (also called stereoscopics or 3-D imaging) refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. These two-dimensional images are then combined in the brain to give the perception of 3-D depth. Besides the technique of freeviewing, which must be learned by the viewer, three strategies have been used to mechanically present different images to each eye: have the viewer wear eyeglasses to combine separate images from two offset sources, have the viewer wear eyeglasses to filter offset images from a single source separated to each eye, or have the lightsource split the images directionally into the viewer's eyes (no glasses required; known as Autostereoscopy).[1]

The Nintendo 3DS produces the illusion of 3D using autostereoscopy. You can imagine the screen as showing two images at once, and having another screen (the parallax barrier) over it to only allow each eye to see one screen. Basically, the screen works by forcing each eye into seeing a different image. The screen has two images; each image is a different perspective of the same image, as if you were actually looking at a real object in real life. In real life, your eyes both see a slightly different perspective of the same object(s), and that provides the 3D effect, as abjects closer to you will have a bigger difference in the perspective. The same goes with the 3DS: it forces each eye into seeing a different perspective. and as a result, your eyes are tricked into seeing objects on the screen just as you see 3D objects in real life. It's all about making your left eye see the left side of the object, and your right eye seeing the right. That's what creates the illusion of depth.

If that's too confusing: it's magic. It's a magic screen.