A ghost note and grace note are not the same thing at all.
A grace note is a very quick note leading into another note, played quickly enough in a way that it doesn't interrupt the rhythm enough to require an actual time value. It can be slowed down enough to the point where it could be a normal note, but that's up to the performer. It can be a slide, hammer, pulloff to the next note, or even both notes picked. It's normally notated as a small 8th note with a little line running through the tail.
A ghost note can have any time value, occupies that exact value (unlike the grace note which doesn't take up an 8th note or anything like that), and is written in parentheses. It's used for a number of purposes, but the general idea is the note is very quiet compared to the other ones. Strumming chords (rhythm slashes) on an acoustic with some soft unaccented notes, on maybe just a couple strings of the chord, an extra note you can hear slightly in a repeat of a phrase that doesn't normally contain the note, open strings ringing sympathetically, soft hits on the snare drum in between the main beats-- all possible uses for a ghost note.
So, grace note = fully audible note, short time; ghost note: barely audible note, can have any duration.