Light wraps - and that means that it, in a sense, sweeps around a subject and has a certain falloff as you move angularly away from the source. To be precise, light is all direct and straight lines but the effect of wrapping is due to the incidence (amount) of light at different angles. So wrapping is effected by the light and the subject.
For example if you have a cube with 1 face directly toward a light there would be no wrapping to the other faces (no light hitting) because there is no straight line from the light to those faces. But with a sphere you would see a fall off around the sphere and the quality of that falloff is dependent on the type of light.
A kino (florescent) light, for example is very soft, so you can think of the light as kinda fluffy and therefore wraps softly and lightly around the subject. However, an HMI is a an extremely sharp hard light (actually a single point light) which therefore has a quick falloff and fast wrap falloff on the subject. Tungsten is also quite sharp, though much less then an HMI. You can make a Tungsten or HMI wrap more by adding some diffusion and thereby softening it.
When placing a light, the softer light will wrap more and therefore will have to be moved further off angle to have a good falloff from light to dark on the subject as opposed to a hard light which can be on a more direct angle. Understanding this will help you to get more bang for your buck with light placement and helps to avoid some flagging you might otherwise do to control a poorly placed light.