It’s more than 170 years since Sir Charles Wheatstone’s first experiments in stereoscopy, which is now the most commonly used technique to create 3D images. The principle, developed by Wheatstone in 1838, of displaying one image for the left eye and another for the right eye to create the illusion of depth is essentially unchanged. If you have a 3D TV, it will use one of two techniques - active or passive - to ensure that each eye sees the image it is meant to see.  Active 3D Televisions that use active 3D glasses work by displaying an image for the left eye and then one for the right eye, alternating in very quick succession. The 3D glasses contain shutters that open and close in sync with the images displayed on the screen to ensure that the left eye sees the image for the left eye and the right eye sees the image for the right eye. Since these televisions display full screen images they are generally able to deliver better quality pictures than passive 3D sets.  Passive 3D The alternative to an active 3D TV is a passive one, so-called because the glasses do not have active shutters to send the correct image to each eye. Instead, passive 3D televisions show one image for the left eye and one for the right at the same time. The 3D glasses have filtered lenses that ensure that each eye sees the right image. The downside of passive 3D is that because two images are being shown onscreen at the same time the resolution of the picture is halved so the images are not in full HD.  3D without glasses The dream for technology companies is to remove the need for glasses entirely. One way of doing this is by using lenticular lenses, which are shaped so that a different image is displayed depending on the viewing angle. Toshiba is using this technique in its glasses-free 3D TVs. However, the largest display is 21 inches - Toshiba says it will take time to make these displays affordable at larger screen sizes. A similar glasses-free 3D effect can be achieved using a parallax barrier, which sits on top of an LCD display to create a 3D effect - this is the approach that Nintendo has taken for the 3DS.
There are many ways to create 3D images. Here is a brief overview of the main    techniques.