There are two types of 3D technology used today:
Passive 3D uses a polarized filter integrated into the screen. Customers must wear polarized glasses that resemble cheap sunglasses – similar to the ones you find in movie theaters – with the left and right lens polarized in different ways so that each individual eye sees only what it's supposed to see.
Active 3D uses “active shutter glasses,” battery-powered glasses with ultrafast LCD "shutters" that open and close in sync with a 3D TV, with the lens for each eye opening at the precise moment that the image intended for that eye appears on the screen. Instead of the TV doing all the work, the glasses themselves do most of the work to decode the 3D image on the screen.
The upside of the active shutter technique is that the glasses allow the viewer to retain the same resolution in the left and right eye with 3D that they have directly with 2D. However, active shutter glasses are much more expensive than polarized glasses, and those rapidly opening-and-closing lenses can lead to a fair amount of flicker.
Polarized, or passive, glasses are a far cheaper and lighter alternative, and there's no distracting flicker. The drawback with polarized glasses, though, is that you effectively see only half the resolution that you'd normally see in a 2D HD image, and on bigger sets, horizontal scan lines can be visible.