more information is needed on our side, spirits can be classified different ways, as well, you might want specif information that we don't know of.
we'll work with the theory they're real [since all the proof comes up with a definite maybe in that department] spirits are energy found floating around the physical plane. [ethral if memory serves] there are many theories as to what are spirits, most common is someone who died but does not wish to cross over for whatever reason. spirits can be human, many meet human spirits, but they can be animal. from experience, these spirits are rather weak, normally orbs, or they will follow a person. they don't move objects or make noise. [typically anyway]
spirits don't typically hang around grave yards of their place of death, unless it was really horrific. another theory for this is because their energy is left behind and imprints on the object or area associated with the death. spirits normally wander until they find a person, place, or object they like and decide to stay. spirits can attach themselves to an object they loved in life, or a home, but normally they just come across a place they like. in my experience, i've met a spirit who stayed in her family home because loved ones still lived their and she enjoyed the company. i also met a spirit randomly one night, he stopped in and decided to stay. if you don't want this to happen, you can cast protection spells on your home.
another thing is energy forms [i think that's the term] sometimes people can create with their own energy a being that would act like a spirit, however, once you stop feeding it the energy, it would go away. spirits also go through stages of alert and dormant, depending on the time of the year [the veil between the worlds grows thicker and thinner throughout the year] and their own energy patterns. some spirits are compelled to do things, for whatever reason, and will go through periods of activity, some could be a short as a week, others could be years. personally, i normally see an average of every 3-4 months.
hopefully that answered some questions. like i said though, if you wanted to know about things like mythology, origin, what they can/can't do and so on, you should be more specific.