A lot of good explenations here....just figured I'd add a little to it.
I play drums and ghost notes, (depending on the type of music, style, and personal preferance) can/are be used a lot. Azrael touched on this, but there are a ton of applications to ghost notes (with both the snare and bass drum)

Take a standard rythm...(hh-high hat; sd-snare bd-bass)
*hope its not too hard to understand*
___1+2+3+4+
hh: X-X-X-X-
sd: -----X---
bd: X-------
*the snare is on the 3rd beat and bass on the 1str

Now lets make this beat a little more interesting....

___1+2+3+4+
hh: X-X-X-X-
sd: ----X---
bd: X--x----

___1+2+3+4+
hh: X-X-X-X-
sd: -x--X---
bd: X-------

In the first example, the bass ghost note comes on the and of 2.
In the second one, the snare ghost note comes on the and of 1.
I use ghost notes a lot when I drum, it's just an easy way (if you can move your limbs independantly) to add a little spark to the beat.
And the ghost notes are ususally played softer, not always on purpose, just kinda happens because a lot of times they come on an up beat. You have to play drums to understand what I mean I guess.