What is the difference between a ghost and a spirit? You can click on the title above to go to the Wikipedia entry for a formal discussion of the terms and the languages and traditions, the etymology of the words, but what really matters is how people use them today.
For most people a ghost is something you see. According to Wikipedia, "the term ghost has been replaced by apparition in parapsychology, because the word ghost is deemed insufficiently precise." This makes sense to me as well. Apparition, from the same root as appear. A ghost, then is what someone might see, an apparition, an image.
So, what is a spirit? Are all ghosts spirits? I don't think so. As I have mentioned in the previous post (New Year, Time Slipping), some ghosts are apparitions seen as part of a residual haunting, but incapable of interaction. They are just images of what once was. They do not possess a spirit. The spirits of the persons involved have passed on.
But sometimes the spirit is present and capable of appearing or speaking or even manipulating objects and energies. This is also referred to as an intelligent haunting. Some people may experience a visit from a loved one who has passed, but others may wander into the territory of a hostile spirit, and this can be unsettling and even harmful. The spirit may or may not be visible (a ghost), but these spirits are just as real as that which animates each of us. As you think and therefore you are, so it goes for the disembodied spirit. It is from these spirits that investigators are occasionally able to get answers to questions in the form of EVP's (Electronic Voice Phenonmenon). (*Note: it's also possible that an EVP may be part of a residual haunting.) Some spirits may also interact with investigators by lowering or raising room temperature or causing a higher energy reading on an EMF (ElectroMagnetic Field) meter.
So, we call upon the friendly spirits to be with us, hope that the spirit of a loved one may join us from time to time, but after seeing an apparition on the stairs, we will likely turn pale and declare, "I think I saw a ghost!"