Ghostgear.com argues that shadow people are completely different from ghosts and they list several reasons for this. They argue that shadow people are too fast moving to be ghosts, shadow people are very solid rather than ephemeral as a ghost would be, and although many ghostly encounters are positive, encounters with shadow people are never positive. They argue that shadow people are probably some form of demon. This seems to be the common belief surrounding the presence of shadow people.
There are also many scientific explanations for the presence of shadow people. The human ability to perceive their environment is enormously complicated and one of the most complicated neurological processes we possess is the process of interpreting things we see only partially. People are capable of seeing only a small fraction of an object and filling in the blanks to understand what the complete object is. Although this seems natural and simple to us, it requires and enormously complicated series of neurological and psychological events to achieve this ability. It is also common for people to use this ability unconsciously, especially when viewing objects out of the corner of their eye in their peripheral vision. For example, it isn't uncommon for someone to see part of a lamp post out of the corner of their eye and think it is an animal or a person. The mistake is quickly rectified when the observer turns to inspect the item. The observer initially only saw a small portion of the object and the mind automatically filled in the blanks, falsely. The same thing could be possible with shadow people. A shadow or passing darkness could be seen out of the corner of the observer's eye and the mind could easily and falsely interpret it as a human shaped monster. This would explain shadow people's quick movements and the fact that they are always seen out of the corner of the eye in the peripheral field of vision where perceptual errors most commonly occur.
However, for those who have and do encounter shadow people on a regular basis, these scientific explanations will never fully account for the terror and dread that is associated with an encounter with a shadow person. So for now, the complete explanation behind shadow people remains hidden in the shadows from which they emerged.