The specular highlight appears on shiny objects. It normally appears on a mirror-like surface. Its appearance is often found on the top of a rounded surface. The incident light hits the object and creates the specular highlight and the light is reflected off the object toward the viewers eye.
As with a normal reflection the angle of incidence is equal to the angle of reflection (See: Specular Reflection). The specular highlight is often seen on a curved surface as a spot. This is because the reflected light bounces from only one spot at the right angle to reflect directly into the recipients eye. As a result, when you see a specular highlight on a curved surface the person next you will probably also see it. However, it will be in a slightly different position on the surface for them as they see a slightly different angle of reflection from their position.
In a perfect case, specular highlights are not seen on the surface of a perfect mirrored surface. In this case they will be seen more as a point of light directly entering the eye. However, a specular incidence of light on an imperfect shiny surface might create a (larger) whitened spot, but not a strong white point. This is because the light hitting an imperfect mirror will scatter. The perfect reflection will not be created. Thus, you might see a bright spot the size of a pinpoint on a rain drop which is a specular highlight. On a billiard ball you would see a highlighted spot that would appear to have diffused and graduated edges. The latter is only a partial reflection, hence the brightness. It is not a perfect, mirrored reflection.
Specular highlights often appear on small curved surfaces like raindrops and for the photographer can create nicely atmospheric sparkles. However, they also appear on uneven surfaces, reflective spots and of a surfaces of mixed materials where some of the constituents are highly reflective small points. Some concretes might have such properties. Quartz or other reflective materials in the concrete create a tiny reflective spot.
Specular highlights are often extremely distracting in a photograph. They readily appear under strong flash and artificial lights. Unfortunately for photographers the bright spots create a burnt out white spot on the photograph. The camera sees these bright spots and is less able to cope with very bright spots than the human eye. So under bright artificial lights they appear when the human eye might not normally see them, or will see a weaker reflection. This causes them to look artificial and stronger than in real life.
A specular highlight sometimes appears to be a spot with diffused edges that gradually get less bright with distance from the brightest spot. This is probably a partly reflective effect (the brightest spot) and a diffuse reflection effect.