A photogenic subject (generally a person), is a subject that usually appears physically attractive or striking in photographs, regardless of their physical appearance in real life. Photogenic drawing, coined by William Fox Talbot, also describes the earliest method for recording camera images.
There are a few different possible causes for this. First, it's important to understand the difference between looking at someone with two eyes as opposed to through a single camera lens. With two eyes, the human brain is able to see the three dimensional aspects of someone's face, even when viewed directly from the front, and it gives much more information than most cameras. With a camera, the subject is viewed through a single lens, and thus much of the three dimensional qualities of the face are lost, and the face may seem narrower, less full, or with different proportions, especially when viewed at a close proximity. An interesting effect can be seen if one compares a close up picture of someone's face to a picture taken from twenty feet away from the same angle (particularly while directly facing the camera). The face will appear different in each picture, and the farther shot will give a better representation of the person's true three dimensional appearance. A more detailed explanation of this concept can be found in the US patent document for the "imaginograph".
Another explanation for the fact that attractive people are not always photogenic is that part of their attractiveness may be due to the charisma they bear in real life due to the way they move, express, and behave themselves. While this will positively influence the subjective appearance of that person in real life, a still photograph usually fails to reproduce these attributes, possibly rendering a picture of the person less attractive than the real-life perception and contributing to classify that person as less photogenic.