So far in both modes of the Screen Space Canvases, we dealt with a UI that was being drawn relative to the entire drawing space of the screen; each of the modes is intended to draw over or against the full screen.
However, in a lot of cases, this is not the desired effect. We want our UI to draw on surfaces, pop up message boxes within (not on) a scene, or even hover over the shoulder of a third-person character (as shown in the aforementioned inventory panel used in games such as Dead Space).
A lot of this is possible today, either using render textures (which require the pro version of Unity), twisting the legacy GUI through code (not very pretty), or using shaders in an intrinsic way (requiring an in-depth knowledge of shaders). All of these are usually outside the comfort zone for beginners or require a lot of time to set up and get right. This is where the new World Space Canvas comes in.