When a player is killed, they're removed from their corpse and transported to the Ankara graveyard as a ghost. At this point they have the option to revive themselves on the spot. However, instant revival comes with the penalty of ten minutes of "Resurrection Sickness", which makes the player weak and vulnerable to attack.
In most situations players will opt to skip instant revival and instead travel back to their corpse in ghost form. As a ghost, no one else can see you, but movement is just the same (you can't walk through walls or anything like that). Once the player is close enough to their body, they can revive themselves with no penatly. Upon reviving they start with 1/2 HP and 1/4 MP.

An unfortunate player begins the walk back to his body
If you log out while in ghost form, instant revival will be forced upon you when you log back in and thus you will suffer ten minutes of Resurrection Sickness. Dying will also carry the penalty of losing 1/4 of your gold, which is why it is wise to put your gold in the bank as often as possible.