This type of portraiture would refer to an image where the face is the predominant element. A typical shot would be where the subject looks directly at the camera, and the photographer concentrates of getting a head-shot which can include part or the whole body in the frame.
This is where the photographers capture spontaneous images, at a given and unexpected moment. Usually the subject is unaware that they are being photographed. This type of portraiture is used typically in photo journalism, travel, street and event photography.
This type of portraiture centres around the subject and the environment they interact with. Unlike Candid Portraits, the subject is aware and acknowledges that someone is taking photos of them but the focus of their attention is not on the photographer but on the people or the environment that they are interacting with. This type of photography is typically used in wedding and family portraiture.