Stereo vision is caused by the fact that each of our eyes sees the world from a different perspective. Our brain processes these two different, although similar, pictures into a single image of the visual world. Thus we can perceive which objects are close and which are more distant.
3D movies are captured with two film or digital cameras which - just like our eyes - are positioned a few centimeters apart from each other, so each camera records a slightly different perspective.
At the theatre, these two images are projected synchronously with two film or video projectors or alternately at high frequency with a suitable D-Cinema projector. To the naked eye the resulting picture is blurred, but with 3D glasses you will see perfectly focused stereoscopic pictures. Each single eye-glass lens lets only pass one of the two projected pictures. Thus the right eye can only see the images recorded by the right camera, and the left eye can only see the “left image”. The brain interprets these two lightly different pictures as one three-dimensioned image, and the events on the screen seem to be almost tangible - a perfect 3D effect!