Also known as an imaginary friend, a shost is a distant cousin to the ghost. Where as ghosts are the lost souls of the dead, ghosts are simply that of the imagination. Once an imaginary person or thing is forgotten of, the "soul" or energy from which the shost is created remains in our dimensional world of "real" and can be seen by everyone, often in the solid state as opposed to the transparent state seen in ghosts.
Due to the often strange looks and behaviors of the majority of shosts, many of them stay hidden in voluntary exhile, though some have been discovered. Bigfoot, Lock Ness Monster, and unicorns are a few "discovered" shosts.
Note: Many shosts are misconstrued due to humams' lack of understanding. The Boogey Man is in fact a nice creature who wishes only to read bed time stories to small children. Unicorns are not so innocent. They attack and kill many animals with their horn for no known reason.
There are no known ways to destroying or killing shosts. Many speculate that they can only be killed by fire, though this has never been confirmed.
The producers should shorten the name to Foster's Home for Shosts.