The first time I had heard this term, I thought it was some kind of new 3D technology. As you can see from the video below, this is nothing like the old 3D world of avatars and blocky graphics.
Augmented reality gives users a portal to view the world in the way they are already comfortable with but overlays it with computer-generated information. In the case of the video above, the iPhone shows you a live video of whatever is in front of you and then overlays information and directions on how to navigate the New York subway system. The effect is incredibly natural (it makes me think of heads-up displays used by fighter pilots) yet contains an incredible amount of accessible information.
Taking this one step further, you could take your iPhone to the mall and watch promotions and discounts get overlaid on the screen as you pan around. Or you could ask for a list of nearby restaurants, and it would flash up the descriptions, ratings and walking directions for each one as you pointed your iPhone in the direction you were headed. The possibilities are amazing.
Note: I think the next hot business plan, probably already being hatched in dozens of places around the world, will be to build a map of the entire world that contains directions, pictures, ratings, advertising opportunities and things we can't even imagine today--all navigable by augmented reality applications. The winning business will allow users and social networks to make real-time updates and comments on, well, everything. For what it's worth, my money is on Google for this, but Twitter and Facebook have showed us that Google's early lead doesn't always guarantee it will dominate as new ideas unfold.