Becoming a ghost (亡霊) is what happens to living creatures when they die. Being dead, one might think that they no longer have any need for food, sleep, physical pain, or other inconveniences of the living condition. Whether any of that is true is currently unknown. What we know is that they like to party, eat, behave cryptically, continue doing the same activities they enjoyed in life, and generally have a good time.
Within the safely guarded boundaries of the Netherworld, ghosts live out the days of their afterlives pursuing whatever happens to come to mind. There are some vague hints about a chain of commands that even the dead must follow, but it doesn't seem like specific details have been worked out. Ghosts are easily distractible.
There ought to be a downside to being dead, but if there is one, it sure isn't obvious.
While Ghosts and Poltergeists are usually thought as same species, the truth is they aren't, according to ZUN.
- Examples: Yuyuko Saigyouji, Youmu Konpaku (only half)