(Source).

- The first is transition bokeh. It’s the blur that results as the dof gradually fades out, when sharp areas transition to being out of focus. But at what point is that out of focus area simply out of focus, and when in the continuous transition does it become bokeh? That’s hard to say. People usually think of bokeh as a blurry area that isn’t easily recognizable as anything in particular, or at least it doesn’t look much like the thing you think it is. So if you examine the above shot of the paint splattered bench, where does the bokeh begin?... You decide.

- Background Bokeh: The second type is background bokeh. Think of an outdoor portrait of a person from the waist up, and an attractive blurry background that looks like it might be leaves and flowers. Portrait photographers often use background bokeh. When done well, it greatly enhances the portrait. In fact, many people tend to think of bokeh as being this background type.

- Foreground Bokeh: However, foreground bokeh is possible too. Some people think it’s too distracting and don’t like it as much as background bokeh. It seems to block the eye’s path to the subject that is in focus. But that’s not necessarily true in all cases. In fact, an eye-catching foreground bokeh might enhance the composition.

- The paint-splattered bench, you’ll often see images involving an in-focus area that gradually transitions away from us into background bokeh, or one that gradually changes to foreground bokeh as it moves towards us, or one that does both. It’s an interesting effect. The in-focus area seems to resolve from or dissolve into the background or foreground blur, or it emerges from a sandwiched position between the background and foreground bokeh.

- Glint Bokeh: The fourth type is glint bokeh. It’s the blurry circles or patches of light from lamps, light bulbs, or small shiny surfaces in the background (and maybe sometimes in the foreground). Distant points of light, especially in a dark scene, light shining though leaves, and specular reflections in daylight often create pleasing bokeh. In fact, when many people talk about bokeh, it’s often this background glint type that they’re referring to.

Stop groaning and read!!!

First of all this is a Japanese word that means diffused, dispersed. It was first used by a Japanese optical engineer to describe the diffusion circles created in camera by any object out of focus in the fore or back ground.

The word was adapted in the west, by photographers, to mean well defined diffusion circles created by light in the background. This definition is not universally accepted and few use the term simply to say 'out of focus' therefore everything not in focus is 'bokeh'.

When looking at this closer bokeh comes in five different flavors (1).

- The first is transition bokeh. It’s the blur that results as the dof gradually fades out, when sharp areas transition to being out of focus. But at what point is that out of focus area simply out of focus, and when in the continuous transition does it become bokeh? That’s hard to say. People usually think of bokeh as a blurry area that isn’t easily recognizable as anything in particular, or at least it doesn’t look much like the thing you think it is. So if you examine the above shot of the paint splattered bench, where does the bokeh begin?... You decide.

- Background Bokeh: The second type is background bokeh. Think of an outdoor portrait of a person from the waist up, and an attractive blurry background that looks like it might be leaves and flowers. Portrait photographers often use background bokeh. When done well, it greatly enhances the portrait. In fact, many people tend to think of bokeh as being this background type.

- Foreground Bokeh: However, foreground bokeh is possible too. Some people think it’s too distracting and don’t like it as much as background bokeh. It seems to block the eye’s path to the subject that is in focus. But that’s not necessarily true in all cases. In fact, an eye-catching foreground bokeh might enhance the composition.

- The paint-splattered bench, you’ll often see images involving an in-focus area that gradually transitions away from us into background bokeh, or one that gradually changes to foreground bokeh as it moves towards us, or one that does both. It’s an interesting effect. The in-focus area seems to resolve from or dissolve into the background or foreground blur, or it emerges from a sandwiched position between the background and foreground bokeh.

- Glint Bokeh: The fourth type is glint bokeh. It’s the blurry circles or patches of light from lamps, light bulbs, or small shiny surfaces in the background (and maybe sometimes in the foreground). Distant points of light, especially in a dark scene, light shining though leaves, and specular reflections in daylight often create pleasing bokeh. In fact, when many people talk about bokeh, it’s often this backgrond glint type that they’re referring to.

Furthermore after researching the subjects you discover that many sites dedicated to Bokeh exists. Folks share their 'bokeh' and that led to a division of 'good' and 'bad' bokeh. The 'good' bokeh is the 'background' bokeh and the 'glint' bokeh. The rest is just bad and discarded as, well, bad photography.

When you look at bokeh from the camera point of view, the fore ground bokeh is formed by a cone of light diffusing away from the lens and as such as no visible center, visible being the keyword. The background bokeh on the other end is a cone of light directed toward the camera and had a specific center in the case of 'glint' bokeh.

Two additional interesting observations: The bokeh stops being visible when it hits the focus plane. The cone formed have angle of diffusion equal to the lens angle. This is why the bokeh is more distinct on a long lens than on a wide angle lens. One more thing... The axes of the cones intersect at the lens nodal point which brings a third observation... Medium range prime lenses will create a more attractive bokeh because their nodal point never moves and is more accurate than a in a zoom lens. The longer lenses flatten everything and render the bokeh somewhat less desirable.

All of this being said, for me and ONLY for me, 'good' bokeh is always formed in the background from any point that reflect light toward the lens under the form of diffusion circle with a visible edge.