The Spectre is a DC comic book character. He was created by Jerry Siegel and Bernard Baily, and his first appearance was in 1940 in More Fun Comics #52.
His story goes a little somethin' like this. There is a being known as The Wrath of God. This being has been an agent of God's punishment and justice since Earth began, and accomplished such holy feats as destroying Sodom and Gomorrah, enacting the plagues of Egypt, etc. The Wrath of God carried on this way until the coming of the Word of God in human form (yeah, that Jesus guy). God declared that Vengeance and Forgiveness could not walk the earth simultaneously, and so The Wrath of God was cast into limbo. After The Word's death, The Wrath burst free. Enraged at his imprisonment, he began to wreak destruction on mankind. God, however, declared that if The Word had been bound to human form, then so would The Wrath.
The Wrath of God sought out a host. Its first was a man named Chakara. Chakara's family had been slain by a being known as Lady Beltane; Chakara's cries for vengance were answered by The Wrath. The Wrath joined with Chakara's soul and formed the being known as The Spectre, part human and part divine vengeance. The Wrath found that, in joining with a human, his powers became subject to that human's will. Each time The Wrath took a new host, The Spectre became a slightly different being.
The first American Spectre was a man named Jim Corrigan. Corrigan was a New York police officer who, along with his wife, was killed by a group of thugs. Instead of dying, however, Corrigan joined with The Wrath of God and took up the mantle of The Spectre (eat your heart out James O'Barr). Corrigan, as The Spectre, fought with many of Earth's greatest foes, until the Time Crisis, Zero Hour, took place. Corrigan failed in his attempt to stop Hal Jordan, former Green Latern, from becoming the being Parallax. Parallax nearly succeeded in his attempt to destroy all of time (he was stopped by Wave Rider and various other superhero-types, but that's another story). Parallax died, but Jordan was resurrected to aid in the prevention of Final Night. Jordan ultimately sacrificed himself to save the Earth from Final Night. As he died, he was approached by (you guessed it) The Wrath of God, who had abandoned Corrigan for his failures. The Wrath joined with Jordan. Jordan, however, because of his many sins, is not yet capable of harnessing all of the powers of The Spectre, and so he has begun to walk the long road to redemption.
The powers of The Spectre are quite vast. It's safe to say that he is the most powerful spiritual entity under the Godhead. He is immortal, invulnerable, and can do lots of cool, miraculous shit. His only weakness (there had to be one) is something called The Spear of Destiny. Yeah, it's a lot like kryptonite to that other guy.
The Spectre deals in big-ass villains, the ones that could snuff out the whole of humanity by blinking. Such baddies include: