A ghost is most commonly defined as the spirit or soul of someone who has died, yet somehow remains on the mortal plane, continuing to haunt a location (such as a house or hotel) which might have been associated with the person in life.
Ghosts have always held some degree of fascination, thanks to holidays like Halloween, as well as the hundreds of books and movies devoted to the subject. In movies like Ghost and The Sixth Sense, the plot focuses on communicating with ghosts, and in real life psychics and mediums claim to be able to do exactly that. With the popularity in recent years of paranormal based reality television shows like Ghost Hunters and Paranormal State, however, Ghosts have become a particularly hot topic in the blogosphere.
On sites like Ghost Research and Ghoststudy, you can view what are claimed to be actual photos of ghosts. Or you can read case studies of hauntings at places like Shadowlands and Ghost Village.