A comic book published by DC Comics. Green Lantern is a superhero who wears a power ring which emits some kind of nifty green energy which can do anything the wearer can imagine.
The first Green Lantern was Alan Scott, who made his initial appearance in All-American Comics #16 in 1940. His power ring was made of a magical artifact called the Starheart -- its only weakness was that it was completely powerless against anything made of wood (which is quite a drawback when you're fighting the evil Dr. Tree). He was a founding member of the Justice Society of America back in the 1940s and is an active member of the current team, as well as one of the people running the Checkmate espionage agency (due to various comic book coincidences, he is still relatively young). He lost an eye during a recent cosmic crisis. For a while, he went by the name of Sentinel, but he recently started using the name Green Lantern again. He is the father of Jade and Obsidian.
Earth's next Green Lantern was a test pilot named Hal Jordan. He was given his power ring by a dying member of the Green Lantern Corps (see Habakkuk's excellent writeup below for a summary of the Corps). His first appearance was in Showcase #22 in 1959. The only weakness of Jordan's ring was that it was powerless against anything colored yellow (again, a big problem when facing the villainy of Captain Pee). For many years, Jordan was the only Green Lantern from Earth, but in later years, Guy Gardner and John Stewart were also given power rings. However, Hal was widely considered to be the greatest member of the Green Lantern Corps ever. But when his hometown was destroyed, Hal went nuts, destroyed the Corps and most of the Guardians, and tried to remake the universe as the supervillain Parallax. He failed and later died restarting the sun when it had been snuffed out by an alien monster. Then he was, bizarrely, chosen to be the human host of the Spectre, God's Spirit of Vengeance.
The last of the Guardians traveled to Earth with the last power ring (and it was the best one, since it was no longer powerless against yellow things). He arrived in an alley and gave the ring to the first guy he saw -- luckily, it was generally nice-guy and freelance artist Kyle Rayner, instead of someone like the Joker. Rayner helped defeat Parallax and continued as the only Green Lantern. Later, when he got hold of some more power rings, Rayner appointed his girlfriend, Jennifer Hayden (the green-skinned and babelicious daughter of Alan Scott), as the newest Green Lantern. Kyle also became, briefly, a god when his ring got supercharged, but he gave up most of that power so he could re-create Oa, the Guardians, and the Central Power Battery. After that, he went into space for several months to try to re-start the entire Green Lantern Corps, while John Stewart stayed behind to be Earth's Green Lantern.
More recently, Hal Jordan was mysteriously returned to life and learned that when he was Parallax, he had actually been possessed by a cosmic horror called Parallax that fed on fear. The Guardians had trapped it in the Central Power Battery, and Parallax's own yellow color had helped cause the rings' weakness to yellow. Parallax was again defeated, and Jordan has resumed his career both as a superhero and as a pilot.