[bmah]

Ghost notes are those that don't correspond to any sound whatsoever (I don't think this would include notes following a trailing legato or pitch bend, but that's still not recommended without distinct sounds going to individual notes).

Ghost notes are typically used as fillers, and are commonplace in pad files, and older keyboard files. You can use them smartly in a few cases, but overall I don't recommend using them for keyboard files. They're almost a necessity in pad files however.