A "ghost" is a very general term for a number of different things. Ghosts can be active, usually unseen entities that inhabit a house or area actively doing things to make themselves known. They were once living but after death for one reason or another have remained on this plane of exsitance. Most are unaware that they have even pasted on. Some are aware but feel they must stay to take care of some unfinished buisness or even to watch over loved ones. Just like us they can be either good or bad. Rarely are they dangerous. It should also be noted that these spirits can be animals as well.
Others ghosts are more of an imprint or recording of an event in time. They repeat this event over and over again, not aware of the present or the people around them. This is called a residual haunting. It is believed that most of the time the event was so tramatic both physically and emotionally that an imprint has been left on this world. These hauntings are not dangerous at all and can be very interesting to experience.
The last type of ghost are what many call demons or evil spirits. These are entitities that never were alive and feel the need to harass the living. These enitities can be dangerous but are extremely rare. You can talk to any paranormal group and they would agree with this. However the practice of satanism or other acts or rituals aimed towards the calling of dark entities can bring one to your home. The inexperienced use of spells, chants, or ouija boards can accidently bring one to you as well. Always use your best judgement.