An earthbound spirit is pretty easy to define. A ghost not as clear cut. They may seem similar but they're not.
An earthbound spirit is here for a number of reasons. They are either trapped here on this plane, not able to cross over because they lack the energy to do so, or they stay here through fear of some kind of punishment if they do cross over, if they were traumatised in anyway like a violent death or if they've just lost their way and don't know how to cross over.
There is a difference between a spirit that has crossed over and comes and visits us, and an earthbound spirit. Just because they are here doesn't mean they are earthbound.
A lot of earthbound spirits are attached to family members. I've encountered them being attached to a person because they knew them in a past life. I've come across earthbound spirits attached to entities from past lives or earthbound spirits who are children attached to a child spirit hanging around us. Lots of variables.
They have no powersource of their own so have to attach to someone or something that has it's own power source.
Ghosts on the other hand are explained in different ways. Personally I see them as an energetic imprint on the land or property. They do the same thing as if they were a broken record. The place that ghosts are seen are areas that held high emotion at some stage in time, perhaps the scene of a trauma. You can't interact with a ghost and it doesn't have a soul or soul facets. Kind of like how if you go into a room after someone has had an arguement and you could cut the air with a knife. The energy has built up over time.
Kate Strong is a psychic/healer and specialises in Soul Healing and Relationship Readings. Please contact her at http://www.katestrong.com