A ghost is a manifestation of some force outside the known laws of nature. A ghost may be a person, animal, or even plant. Ghosts can be friendly or hostile, and they can communicate through various means.
The character that the ghost appears to should be a person that your protagonist knows or is related to (or has some other relationship with). That way, the reader will feel more compelled to follow the story, as they will be invested in the ghost’s behavior.
If you are writing the ghost story in first person, then the ghost is probably the protagonist’s deceased sister or best friend. A good ghost story will have the ghost interact with the protagonist a lot, so the reader can learn more about the ghost. A good technique is to frame the ghost through the protagonist’s memories, responses, fears and imaginings.
Depending on tradition, a ghost can appear as a translucent figure, walk through walls, enter the body of another, manipulate objects and take on a physical form. It is important to let the reader know what a ghost can and cannot do so they will be prepared for it when reading your story.
Ghosts can be seen as caricatures of the person they once were. This is a great technique for romance, paranormal and thriller/suspense stories. However, ghosts should still have a name, appearance and personality. They should act and behave in the same ways as all other characters in your story – it is just that they are dead.