"You suddenly realize it is unnaturally quiet
on the back of your neck
"You have an uncanny
"Run away! Run away!"
Indeed, these are the NetHack messages that you sometimes get as you walk around on a level that contains a graveyard. They have a 1/6 chance of appearing on any level past Dungeon level 11.
In a graveyard, about 1/5 of the squares will be occupied by corpses, 1/10 will contain chests or boxes, and another 1/5 will be graves. Graveyards are filled with sleeping monsters from classes ZV&, wraiths, and ghosts; the precise composition varies according to depth. Graveyards generate ambient messages as listed above. You only get the "Run away! Run away!" message if you enter the graveyard between 0:00 and 0:59 (Midnight hour).
Graveyards aren't terribly useful to have around. True, they provide abundant wraiths, but a wraith has a much reduced chance of leaving his precious corpse on a level that contains a graveyard, and it's quite a bit of trouble to manipulate several wraiths at once to follow you to other Dungeon levels. Furthermore, the concentration of creatures that may have energy drain and/or demon summoning powers can be very dangerous. Of course, graves do provide amusing messages when excavated, but I don't think they're really worth the bother.