What is AR?
AR was originally developed in the early 90s but the recent advances in technology and proliferation of smart phone have made its use more viable. Early uses of AR were in marketing and advertising but it is increasingly being used in other areas, including education.
On a technical level, there are broadly two categories: Marker-based, and markerless systems.
Marker-based systems or natural feature tracking use physical-world symbols and act as a reference point for computer graphics to be overlaid. So I could take a 2-dimensional printed marker, point my mobile phone at the marker and my phone would interpret this symbol to overlay an on-screen graphic as if it were directly on top of the marker in the physical world.
An alternative approach is use of markerless systems which use location data or GPS to determine the position in the physical world. The location data can them be compared to a database to determine what the device is looking at, and influence what will be displayed on-screen.