The aim of augmented reality is to bring graphics, audio etc. to the real world in real time. For example for apps on a smartphone are two different approaches in use, the marker-based or image recognition and location or GPS-based approach.
Marker-based applications are using a camera to overlay the image with content or information. This is happening in real time. The camera feed is processed and the real image is augmented by some content or information. Markers are for example QR codes. If you are scanning an QR-code and you are getting additional information on your screen than you are using a marker-based augmented reality application. Marker based applications are most common in augemented reality because it is much more simple to recognize something what is hard coded in your application.
For example Nissan made a application where you can see one of their cars in augmented reality by just using your webcam and a printed wheel. The demo is under the following link:
Location-based applications are using the GPS tools in the device to determine the users position. The position and the direction of the user are the input parameter for the app to show at what the user is looking at.