ANSWER: A ghost is conscious being (human or animal) that no longer has a physical body. Unlike spirit guides, ghosts are generally thought of as not having fully completed their transition to the afterlife, nor have they yet done their life reviews. Because of this, they tend to have the same personality and beliefs as they had while living. They aren't smarter or more enlightened. But neither are they necessarily evil--although they certainly can get frustrated or sad for being ignored. Let's look at some questions that I'm often asked about ghosts.
1. Why do ghosts look like they do? How you see psychic information (which is how you see ghosts, not really with your eyes) depends on two things -- the sender and the receiver.
SENDER: Ghosts can appear in different forms and as different ages depending on how they feel that day or what kind of impression they want to make on you. They tend to reflect their self image in obvious ways, as their actual appearance. Their clothes (ever wonder where ghosts get them?), age, shape, and form, can be reflections of how they feel about themselves.
RECEIVER: Your unconscious mind acts as the filter for psychic information. It literally colors the image. Thus, the color, shape, size, form, and even temperature of the ghost can be your personal code for the ghosts, rather than a literal depiction of them. It is this aspect, the personal filter, which often screws people up because it allows your own biases, wants, needs, and desires to warp the psychic information that you receive. To some extent, it makes understanding yourself - your own personal wants, needs, desires, and issues - important for you to look at. The more a person understands themselves and is on good terms with their unconscious mind, the more accurately they are able to perceive the information they receive psychically.
2. How can I tell the difference between a ghost and a place memory? It's easy. Just pay attention to whether it interacts with you or simply repeats the same actions over and over. It is useful to think of a haunting case as being where the environment simply records strong impressions (either from highly charged or frequently repeated events). Sensitive people may be able to pick up on these recordings, and see or hear events from the past (but not necessarily from the dead -- the living can make these recordings, too). With a haunting you see reruns, over and over. A ghost is different. A "ghost" can sometimes see or react to your presence -- and its not uncommon that they are the one running away in fear. Ghosts generally don't want to harm you. Some are lost, some confused, some have messages they want to pass on, and some just like hanging out.
3. Can ghosts hurt me? It is rare (though not impossible) for a ghost to be able to hurt you. Why? Because you have everything a ghost has and more. You have a body. Not only do you have just as much mind/soul/spirit as they do, but you have the advantage of being able to do things physically. It's pretty much true that they can't hurt you unless you think they can (with only rare exceptions, involving mind-matter interactions) ... Loyd Auerbach has had many encounters with ghosts, and has humorously commented that you have to remember ghosts are people too. He's been known to use such drastic measures as telling hours of knock-knock jokes (guaranteed to send ANY being, living or dead away screaming, vowing never to return) or singing "It's a Small World, After All" over, and over, and over. They are like people in having personalities, likes, and dislikes.
4. What should I do about a ghost? What you should do, if anything, depends a lot on how you feel about the ghost. It's somewhat like a haunting in that respect. You might enjoy it or ignore it if it isn't bothering you and the ghost seems happy. However, I think the ethical thing is to often consider the ghost's needs, too. Are they confused? Do they need you to explain to them that it is time to move on to the light? Do they need help moving to the light? After all, if you believe in reincarnation, it may not be in their best interest to remain stuck as a ghost. If you want to move them on to the light, there are many ways to do so (some mentioned above). The simplest is to imagine the tunnel of light opening up, and/or friends and loved ones coming to help the ghost move on. Of course, if it's a spirit guide, don't expect it to go anywhere. It's supposed to be around.
There is more information on spirits, the adjustment problems they can have, how you can communicate with them, and ways of moving them to the light in my book, Handbook to the Afterlife.