As is usually the case, things are much more complicted than that. There are many cases of ghosts that look and behave like normal ghosts, but they are the ghost of a person who is very much alive. Ghosts of inanimate objects such as coaches and trains are reported. Ghosts of imaginary people have even been created by groups of determined investigators.
Most people use the word ghost to refer to something that is seen but has no material existance.
Ghosts don't usually make any sound and don't affect the material world. If you accept the more general definition of a ghost, there are many theories as to what they are. The obvious explanation, but as stated above cannot apply to all ghosts. It is very hard to prove or disprove. There are psychics who claim to be able to communicate with ghosts and help them to move on to the next stage of existance.