By synchronizing them to the content on the HDTV, full-resolution 1080p images can be presented sequentially to the left and right eyes, and still have a total frame rate of 60Hz.
The glasses sync to the display via infrared - there's a transmitter on the bottom of the display panel and a receiver in the glasses.
The main disadvantage of this shutter approach is that the active glasses need a battery.
Passive Polarised Glasses work much like the old red and green glasses, but using polarised filters rather than red/green means you get a full colour experience.
It means cheap, passive glasses but complicated and expensive screens and projectors. If you've seen a colour 3D movie, this was probably the way it was delivered.
Autostereoscopic display is a name for a screen which displays 3D without needing glasses by use of a lenticular or 'parallax barrier' layer in front of a specialised (usually LCD) display, presenting a different image based on viewing position.
No glasses, but a very limited viewing angle.
