3D glasses work by providing a separate image to each eye. The brain then combines the two images into a single image with 3D characteristics. The 3D process fools your brain into thinking it is seeing a 3D image, so it creates one for you.
3D-enabled TVs and 3D-enabled Video Projectors work by incoming 3D signal that can be encoded and sent in several different ways. The TV or projector has an internal decoder that takes the 3D signal and displays the left and right eye information on the TV screen in such a way that it appears to look like two overlapping images that look slightly out of focus
One image is intended to be seen only by the left eye, while the other image is intended to only be seen by the right eye. In order to view this image properly, the viewer must wear glasses that are specially designed to send the left and right eye images properly to the left and right eye. When viewing the overlapping images through these special glasses, the image appears to be in 3D.