Halo highlights or zenith highlighting for me is when the light source is overhead.
You generally have only one light source as well with this method. It creates consistent highlights and shadows across the whole model that are representative of real life.
In this case, not every edge will be highlighted, only those that are hit by the light source the rest of the edges will be in shadow.
Overall highlighting is when you highlight all of
th edges on a model no matter where they may be and whether or not light can hit them or not. This is more to define all the shapes on a model and not so much to make it look as though it's "real" like the zenith approach does.
Both of these are different from Object Source lighting where something on the model is "glowing" and casts light on other parts of the model. That's completely different.
I do have a post on zenith highlighting that might help explain it more:
http://fromthewarp.blogspot.com/2011/09/how-to-do-basic-zenithal-highlighting.html