there's an old jewish tale about lilith [before eve, adam was with lilith, look it up] anyway, adam and lilith had a bunch of kids, then she was banished and eve came in and her and adam had kids. one day god came down to eden to inspect, and eve convinced adam to hide liliths children, and he did, when god asked to see all of adams children, only eves kids were present before the lord, and when he asked if there was any more they said no so god said, may all the unseen childen never been seen again. thats how the jewish believe there's demons in the word anyway, i like that story personally. [always been a fan of the old testament stories]
demons are like us only from another plain of existence. they have feelings, they're not good nor evil [typically though they're malicious and love messing with people] some are more powerful than others. in much the way some humans are best at athletics or science, or art, demons are skilled in certain ways, some are old and some are young, and over time they get stronger if they weren't born strong.
ghosts [which i know more about] there are a couple types, a ghost can be a person or animal that has died in such a way as to make them want to stay. [they don't feel it was their time, they have a strong connection with someone or something, they died by accent like fire or a crash] when this happens, they're either classified as a ghost or poltergeist. a ghost is normally attached to an object or place, where a poltergeist is attached to a person, not necessarily a creepy little girl. ghosts are old so they can be whatever age they feel if they're strong enough to materialize. i've dealt with a few ghost, luckily no evil ones, some are just strong enough to be, so while some people/animals can sense them, they just float around not doing much, and normally cross over. others are attached to a person or object and won't make themselves known until said thing is in danger and needs protecting, the third type is highly active.
spirits are another thing entirely, some ghosts are dead people/animals but were born spirits, like demons, and are from another plain. they are powerful, but they can't really cross over because as i've said, they've always been this way. they can get weak and 'die' but for the most part they're immortal.
ghosts and spirits tend to go through cycles of active and inactive, it can be a span of weeks, months or years. Rori [my spirit companion] usually is extremely active for a month, then inactive for about 4. what makes a spirit, ghost or demon strong, i'm not too sure of with all of them, certain entities 'feed' of things. for instance, many shadow men feed off of fear, the more someone is afraid, the stronger they get. my friend has one in her room, but he feeds off joy. most ghosts and spirits though rest to regain strength. demons i'm not so sure on, i think unless weakened they're always about the same strength.
in regards to ghost hunting shows, i'm not a fan of them [unless it's true story documentaries or involve B list actors like gilbert gottfried] because they're usually fake. or if they aren't, think about how you would feel, a bunch of strange people barge into your house at night, set up camera's and lights, split up, and demand you entertain them? would you happily greet them with tea, or go Home Alone on them?