Since I doubt this will be fully explained in the LP, I suppose I should mention something about how ghosts work. Ghosts are the result of a dead dwarf not being properly buried or not having a memorial made for them. However, just because a ghost is not buried does not mean they will become a ghost. The current mood of a dwarf upon dead influences the chance of them rising from the dead. An unhappy dwarf is far more likely to become a restless haunt than a happy one.
You may also have noticed there are different types of ghosts. The circumstances surrounding a dwarves death will also influence the type of ghost they become. Different ghosts do different things, ranging from minor annoyances (spooking) to incredibly dangerous (murdering the living). Also, ghosts are not restricted to residents of your fort. Foreign dwarves, such as merchants, can also become ghosts upon death.
If the body of a dwarf isn't available to be buried (fell in magma), you can use a memorial slab instead. Memorial slabs are engraved with the dwarves name, along with some info about them based on their personality ("Here lies Urist, lover of oysters"). However, the names of nondwarves can be engraved as well. Even legendary creatures can be receive engraved slabs, suggesting that Toady planned for ghosts to go beyond dwarves at one point.