This essay will relate Ghost Pokémon with Psychic types and Dark types.
Ghost Pokémon are a great mystery, not only to Pokémon scientists but also to Pokémon fans. Are they sentient gasses or dead Pokémon? What is the extent of their powers? Let us look at the available evidence.
First, in the PokéDex, Ghost Pokémon such as Gastly are said to be "gas" Pokémon; in other words, living gaseous entities. They also apparently inhabit noxious areas. Later generations of Ghost Pokémon such as Duskull and Misdreavus do not entertain such a notion, so it may be that the Gastly line is unique. Perhaps there is some extra-dimensional being- a gaseous being -that appears in the Pokémon world. Perhaps they are also related to the Unown, who also live in another dimension and can alter reality as they see fit (Isn't it strange that all Ghost types are purple-ish, and the vortex the Unown come from is purple-ish? Hmm...). As Gastly and Haunter have shown, they can have indeterminate shape until properly revealed in some way, either with the Silph Scope or with a move such as Foresight. It is quite possible that these sentient gases, being known for mischief and deceit, take on the form of ghostly terror common in a certain locale (vampire, grim reaper, disembodied head, gremlin, etc.) to take advantage of people's fright. Later Pokémon such as Shuppet appear to feed off negative emotions of humans and/or Pokémon, and the Gastly line can steal one's life force, which may be saying the same thing. Perhaps, as a gas, they are unable to maintain their favored form unless supplied with energy from solid living beings. By extracting some unseen energy from people or Pokémon, they acquire the atomic bonds necessary to gain greater cohesion.
Second, Ghost Pokémon could literally be just that- ghosts, or, specters of deceased Pokémon. After all, they appear as ghosts, phase in and out like ghosts, regularly inhabit cemeteries and other abandoned dark buildings, and they have the same playful or sinister attitudes that our understanding of ghosts share. The Gastly from the Stone Maiden's Peak, for example, stated that it could visit the spiritual realm. The ghost Pokémon aboard the haunted pirate ship in the Orange Island saga maintained watch over their deceased master's ship. In the games, particularly the first generation, ghosts are present in Lavender Tower, a cemetery for Pokémon that is haunted by the ghost of a Pokémon poached by Team Rocket.
However, something isn't quite right about seeing these Pokémon as real ghosts. The ghost of Marowak in Red, Blue and Yellow, for example, was revealed in the game as a Marowak, not a Gastly or Gengar. Supposedly, real ghosts maintain their forms (or have demonic-looking versions of their living bodies): stone maiden, Ambertwo and the ghost of Marowak. I think the real reason Ghost Pokémon frequent cemeteries and other unpleasant areas is not that they are deceased Pokémon, but they are Pokémon searching for negative emotions on which to feed, and there will always be plenty to be found in such dreary surroundings.
Finally, Ghost Pokémon seem to have a wide variety of tricks to pull on others. Most Ghost Pokémon in the anime at least accomplish the hypnotizing of unwary travelers, creating poltergeist phenomena in buildings and grossing out people and Pokémon with cartoony gross depictions (eyes bulging out, disappearing except for their toothy grins, etc.). Yet, some seem very powerful and sophisticated. The Gastly of Stone Maiden's Peak was able to pose as a human, a human ghost, an animal (a mongoose, if memory serves) and even create a hybridized Pokémon that does not exist in either game or anime reality. The ghosts of the pirate ship, meanwhile, appear to share Latios's sight-sharing ability, allowing others to see events as they remember them in an almost holographic manner. There does not seem to be a limit to a Ghost Pokémon's creativity.
Psychic Pokémon also seem to share the ability to look into the minds of others, not to feed, but to communicate. Possibly the closest descendants of Mew on the evolutionary chain (At least the Abra line, since it shares some characteristics with Mew), Psychics differentiated over time into a multitude of forms, such as Jirachi, Latios, Latias and others. It is also possible that only a few Psychic Pokémon actually share genetic ties with the origin, since some Pokémon, such as Deoxys, presumably came from space, and thus is independent of Earth's evolutionary history. In any case, Ghost Pokémon can damage Psychic Pokémon greatly. It must take a considerable amount of concentration to read minds, move objects telekinetically, etc. A Ghost Pokémon's ability to prey upon one's thoughts for nourishment must disrupt a Psychic's command of atomic forces (read The Science of X-Men by Yaco and Haber to get a good grasp of how to be telepathic and telekinetic).
Dark Pokémon, too, provide obstacles for Psychic Pokémon. Apparently evolving around the Saffron City area at first (Since that is where one acquires them in Generation II.) in response to the tremendous psychic presence of Sabrina and her Psychic Pokémon, Dark Pokémon developed a means of immunity against Psychic attacks. Presumably, Dark Pokémon disrupt the neural pathways of Psychics, rendering them ineffective; or, they simply lack the neural mechanisms present in all other beings that would put them at a Psychic's mercy. As darkness is just the absence of light, this latter explanation seems more reasonable.