Ghost Features (3.5e Variant Rule)
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Ghosts usually have powers that related to the way they died- ghost who been burned on the stake might has some amount of Pyrokinesis, a ghost who froze to death might has some amount of Cryokinesis, a ghost who drown might has some amount of Hydrokinesis, and etc.
Plus, if ghost had some supernatural powers while it was alive, they most likely grow stronger (telekinesis became greater telekinesis, dimension door became teleport (without mishap chance) and etc.).
As rule of thumb, vengeful ghosts (ghosts who stay on earth for revenge, usually on the ones who killed it or the ones it sees as responsible to its death) are more powerful than normal ghosts, and violent ghosts (ghosts of people like serial killer who executed and etc.) are even more powerful than vengeful ghosts- the power of ghost is in direct line to its emotional state: emotions like rage, anger and vengefulness will majorly empower the ghost.
Banishing or Destroying Ghosts
Obviously, it is difficult to kill what is already dead. However, there are a few ways to take care of problematic ghosts.
Most ghosts can't touch iron (cold iron, and other metals that acts like it, are just as useful for that matter) and salt. Direct touch with either will abolish their manifestation for a while (between 1 minute to 1 round, depends how powerful the ghost is), or force them to leave a possessed body. They also can't walk on holy ground or they will be destroyed (powerful ghosts and ghosts of clerics are immune to that effect). Ghost can be forced to move on by burning its remains. However, some ghosts attach themselves to objects who were important to them when they were alive. In such cases, those objects needed to be burned too to force the ghost to move on.
Some powerful ghost can be partially resillient to these effects, however the only kind of ghosts that completely immune to them are Living Ghosts, because in the process of becoming living ghosts they partly resurrected, enough to make them more alive than other ghosts are (and therefore, immune to ghostly weaknesses), but not enough to make them truely alive. They are even immune to burning their remains, as they are the only ghosts who attached to earth on their own.
However, all ghosts have one common weakness- other ghosts. Ghosts can destroy weaker ghosts by absorbing their energy, which forcing them to move on and leaving them so damaged they would never could be resurrected using mortal's magic (and characters with divine ranks have only 5% per divine rank they have to succeed). If two ghosts fight, the weaker ghost will be affected similarly to what explained above. If the two are equal in power this will happen to both of them.
The only way to actually destroy ghosts for good is to use a method that destroys the soul (as ghosts are, in fact, souls with baggage).
Ghosts of Outsiders
While moat outsiders (except Native ones) can't become ghosts after death, sometimes they are cursed to become ghosts by transforming them into ones while they are still alive. However, only powerful (and mostly malevolent) spellcasters can do so, and some Necromancers sees it as a good way to acquire powerful, yet resentful, servants.
Ghosts that created that way are immune to burning their remains (since there are no remains to burn- their body transformed) and might have resistance, or even immunity, to other ghostly weaknesses, depends how powerful they were before they cursed. However, since most outsiders are powerful compare to other creatures, outsiders that cursed to become ghosts are usually more powerful than the common ghost, and can have some unique powers and abilities.