For those who have never heard of "augmented reality" (AR), here is Wikipedia's definition:
Augmented reality (AR) is a term for a live direct or indirect view of a physical real-world environment whose elements are merged with (or augmented by) virtual computer-generated imagery - creating a mixed reality.
Sounds a bit blury? Well... I'll try to make it clearer with a demo...
First, you will have to print a little black and white "marker". The AR software scans the webcam picture and look for this very marker. When it is found, its 3D position, rotation and scale are computed and used to embed a 3D object. You can found the marker here and it looks like this: