The Dark type is one of the eighteen official types from the Pokémon franchise. It marked its debut in the Generation 2 games and was specifically created to tone down the Psychic type's prowess in order to make the Pokémon games more fair. Dark is an eccentric sneaky type, with most of its attacks often having a side effect, as well as being centered around dirty tricks.
Dark moves seem to work most effectively against Ghost and Psychic Pokémon, but not so much against Dark itself. The Dark type is also resisted by Fighting and Fairy-type Pokémon. These same types are the Dark type's very weaknesses as well. Ghost is also resisted by Dark, and the Dark type is even immune to Psychic.
Prior to Generation 6, the Dark type's resistances complimented the Ghost type's weaknesses and vice versa. This resulted into Pokémon with this type combination (Sableye and Spiritomb) lacking any weaknesses, because one of the types resisted or was immune to what the other type was weak to. However, 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 45 official Dark-type Pokémon. 11 of those are pure Dark-type. Concerning dual-typed Pokémon, Dark Pokémon are about as much primary as secondary typed, with only a slight difference of 3 between the two sides.
Dark-type Pokémon are centered around sneaky creatures using dirty tricks to survive, or sometimes being downright brutal. Some Dark-type Pokémon are literally based around darkness and/or are nocturnal. Since this kind of behavior isn't exclusive to a certain kind of organism, Dark-type Pokémon can be pretty much found in any Egg Group, with its overall appearance and other type (if it has one) determining exactly what Egg Group (e.g. Shiftry, being Grass/Dark, can be found in the Grass Egg Group, while Hydreigon, being Dark/Dragon, is found in the Dragon Egg Group).