Ghouls are slimy, buck-toothed, ugly creatures which live in attics or barns of wizards. They are dim-witted and are content to throw things around now and then. Ghouls are relatively harmless creatures who live on spiders and moths. A ghoul lives in the attic of the Burrow, just above Ron's room. The ghoul makes noises a lot.
So I guess that ghouls have a more solid form than ghosts do. Anyway, I would assume that only wizards become ghosts. Like Mint said, it would be incredibly confusing to die and find yourself in a world filled with magic! I did read an interview where JKR said that Book 5 would reveal what makes some wizards turn into ghosts. 49 more days...49 more days...[/quote]