Definitely believe in ghosts, or spirits(souls) if you will. To me they are all the same thing. My take on it is, they are among us constantly, everyday, everywhere. They are trapped here, existing on a alternate plane (or dimension), by the vices or misdeeds they preformed in their mortals lives. They are forced to re-experience those misdeeds for the rest of eternity. Some misdeeds are more severe than others, of course, thus the varying levels of malevolence found in ghosts, due to the suffering they have to endure. In other words, this dimension is what we would call "hell", although I don't believe that the devil or his demons exist there. That is another dimension entirely, although that is not to say they can't visit.
The reason we see ghosts from time to time is based on two things. They want to be seen, and we are receptive to seeing them, meaning we have they ability due to state of the unconscious mind being "in tune" with the ethereal projections these spirits are able to do. They know who can see them, and may or may not allow us to see them. The same is true for sounds or voices from this dimension.
As a side note, all living things have a spirit. Every animal, every leaf, every blade of grass has a ethereal equivalent which is why sometimes we see deceased pets. They, too, are trapped because they don't have higher intelligence and understanding that humans have. They don't suffer like their human counterparts do, because they are not governed by the higher power we call God and therefore are not subject to the same punishments.
Just my 2 cents.
