For the uninitiated, augmented reality typically uses GPS location and internet data to layer information on the screen over live video. In Layar's case, for example, to show local search results for "pizza" over the restaurants in an intersection you're looking at on your iPhone.
Whether cool translates into usefulness -- we'll see. But on cool alone, it's definitely worth a look. (You can also view search results in map and list view, but what's the fun in that?"