The Ghost type is one of the eighteen official types from the Pokémon franchise. Ghost is certainly an odd type that is in overall well to use offensively, due to all-around good attacks and a bunch of moves with an effect or status condition as a cause. While it is resisted by Dark and even did no damage to Normal at all, Ghost moves are super effective against the ghost type itself, making it one of the only two types that is weak against itself (the other being Dragon).
Above all, the Ghost type also is strong against Psychic. However, in Generation 1, the Ghost type was programmed incorrectly, causing Psychic to be immune to Ghost instead. Since the only Ghost-type Pokémon that were available during Generation 1 was Gengar and itspre-evolutions, as well as Psychic being super effective against Poison, Ghost-type Pokémon weren't the best choice. As of Generation 2, this is fixed.
Defensively, the Ghost type resists Bug and Poison very well, and is even immune to the aforementioned Normal type, as well as Fighting. However, aside from Ghost itself, they were also weak against Dark.
Prior to Generation 6, the Ghost type's resistances complimented the Dark type's weaknesses and vice versa. This leaves Pokémon with such type combination (Sableye and Spiritomb) completely without any weaknesses, because one of the types resisted or was immune to what the other type was weak to. With the introduction of the Fairy type, Pokémon of this combination now have a weakness, since Dark is weak to it, and Ghost doesn't have the resistance against Fairy.
As of Generation 6, there are about 34 Ghost-type Pokémon, making it the single rarest Pokémon type, beating out Fairy, Ice and Dragon. Only 10 of these are pure Ghost-type. Of the dual-typed Ghost Pokémon, 15 are primary Ghost and 12 are secondary Ghost.
The Ghost type centers around specters, apparitions, the undead, and other spirits of spookiness. As such, there are Ghost Pokémon based on a certain legend here or there, such as Spiritomb being a Jibakurei, or a Gourgeist being a Jack-o'-lantern. Many Ghost Pokémon are part of the Amorphous Egg Group, as for being ghost that don't have a clear shape. There are Ghost Pokémon that appear in the Mineral Egg Group instead, possibly for housing an object (thus giving them a determinate form), like Shedinja or Honedge.