A talking head is another term for interview. A person on-camera speaking directly into the camera is a talking-head. To spice up a talking head, you cover the head with b-roll and turn it into a voice-over.
B-roll is a term indicating the video pictures used to illustrate a story. If you do a cooking show, interview a chef and edit it together with video of him preparing his specialty, the video of him cooking is known as B-roll. The interview video is NOT B-roll, it is known as the A-roll but that term is never used anymore. B-roll is an extremely common term. Any video you take OTHER than interview footage is known as B-roll.
A voice over is another word for narration, or anyone speaking on camera without being seen. The voice track is laid over the b-roll.