Ghosts
When people talk about a ghost, this is an imprint in time, something that is connected to a building, like a video playing over and over again, so if you wake up in the night and there is an old Gentleman sitting at the end of your bed who is continuously doing the same thing but does not and can not interact with you, then this is a Ghost. A ghost cannot hurt you and will not talk to you or touch you, it may be alarming to see at first but it really is nothing to worry about.
Spirits
If you are looking to attend a Paranormal investigation then it is great to see Ghosts from above but ideally what you would be aiming to find is a Spirit. A Spirit can interact with you, touch you, move things, make noises etc. they require a lot of energy for this which they tend to get from you and anything electrical, a spirit can show itself but it is very rare to get a full manifestation. They normally show themselves in light forms, mists and orbs the closer the orb is to red the more likely the spirit is about to manifest itself in front of you.
Poltergeist is just a spirit that is more malicious than a normal spirit and will be more inclined to throw things at you and try to harm you, these spirits are the ones more likely to need clearing from you home.