Wow, contreversial opinions coming here then ha ha ha!!!
My opinion (not an expert at all) is that a ghost is our spirit after it has died, i believe somehow that our world just mirrors or exists alongside another and sometimes our energies are that strong we can revisit special places or special events that we remember and sometimes if the conditions are correct we can catch a glimpse of that spirit..
I never have felt a ghost is to be thought of as nasty or evil
A ghost is (just my theory) a recording or an imprint of a person or being that resided on earth, we all give off an energy so makes sense for that energy to imprint on reality kind of like a sound does on a recording. Makes sense for us to witness that recording, typically a ghost is seen harmlessly going about its business so hence the theory it is simply a recording as there is no interaction or seemingly any awareness we are near them