It's sounds like science fiction, but “augmented reality” is real technology that superimposes computer images and some audio over the real world in real time. It's sort of like the POV of Iron Man we saw in the movies.
If you're a football or hockey fan, you saw the tech used to show how far up the field your team needed to go for a first down, and as colored trails that followed the pucks shooting across the ice. This technology enhances your view of the world with computer images and text. You're laying real-time digital information over the real world. The U.S. Postal service is using it to help customers use their webcams to find the right sized shipping box. BMW mechanics wear special glasses that provide an enhanced view of a car engine, and play audio instructions through a set of integrated headphones. (Very Terminator.)
The Layar Reality Browser, for example, combines real-time computer data with geo tagging information from location-sharing service Brightkite, on images you see of the real world through your camera phone. The camera displays the view, but also superimposed photos, location markers, and real-time comments from other uses in the area. You can even see real estate listings for the nearby buildings.
The opinion-sharing service, Yelp, supports an iPhone application called Monocle, which combines the phone's camera view with the tags that identify landmarks, bars, and restaurants in that view, and pairs them with Yelp's user reviews. We're at the bleeding edge with this technology, but it is too cool not to go mainstream, fast.