ARToolKit for Unity Introduction
From ARToolworks support library
What is ARToolKit for Unity?
ARToolKit for Unity is a plugin for the Unity game engine that integrate's ARToolKit's augmented reality tracking with Unity's graphical and game development features.
Essentially what the plugin does is align a virtual camera within Unity with a real-world camera (such as a webcam) relative to a tracked marker target. For example, if you print a marker on a piece of paper and point your webcam at it, the corresponding virtual camera in Unity will "look" at the equivalent spot in the virtual world. If you then place a 3D model in the virtual world, and overlay it on the incoming video, you produce an augmented reality view.
While many such applications and toolkits provide this functionality, using the Unity game engine is appealing due to its extensive feature set, cross-platform operation, ease of use, and relatively low price compared to other offerings.