The Dark type was added in Generation II to cause a weakness on Psychic types, which had the weaknesses to Bug type (the moves were really weak to cause a major damage) and Ghost type (the only move was Night Shade).
Many Dark type Pokémon tend to be aggresive, mysterious, malicious, cruel and clever, but a Dark-type Pokémon doesn't have to necessarily be evil, like Absol.
It's hard to a Dark-type Pokémon to form bonds of friendship with its Trainer, but when having one, it will never break, being really faithful to its Trainer and protecting it anytime.
The Dark-type Pokémon, along with the Poison types, are mostly used by the criminal organizations.
They're strong against the Psychic type because of the fears, and the fears can affect one's rationality and concentration.
They're strong against the Ghost type because evil can damage the soul.
They're weak against the Fighting type because in the philosophy, a warrior must overcome their fears, represented by Dark. Also it can be said that the Fighting type represents justice, the opposite of what the Dark type does.
They're weak against the Bug type because bugs operate well in darkness.
They're weak against the Fairy type because fairies represent light and the darkness disappears with light.
If a Dual-type Pokémon is both Dark and Ghost type, both types cancel out each other's weaknesses with their resistances, leaving Pokémon like Spiritomb and Sableye with no weakness apart from Fairy.
This is the only Pokémon type not to be used in a Pokémon Gym.