Ronnie James Dio introduced the raised index and little finger sign to metal world in the 1980s. It was quickly adopted by other metal bands and somewhat identified as "heavy metal sign".
Satanists also use it in their rituals to represents the horned head of the Devil, the Satan and as a sign of recognition between those that are in the occult.
It has a much older and stronger meaning too. For example, in the ancient Egypt, the sign was called the evil-eye sign. It was used for magical protection against the "evil eye". Many of the pharaos, buried in the pyramids, had sculptures guarding them which often showed this sign to guard the pharao and to curse the grave robbers.
The sign also has many other meanings, depending on the culture and region where it is shown. It is refferred to as "The Horned Hand" (Mano Cornuto) by some religious groups.
(Google for "Horned Hand", "Mano Cornuto" and "Evil Eye". Tip: Some Info on the Mano Cornuto (Horned Hand) and the Evil Eye)