Dignan,
There have always been multiple Green Lanterns. The Green Lantern Corps were an interstellar peacekeeping force, much like the Jedi Knights, and they all had the magic rings as their weapons.
Alan Scott was the first Green Lantern on Earth, in the 1940s. He was not affiliated with the GL Corps, and he got his ring from some magical force called the Starheart. He wore red and purple, joined the Justice Society of America (which predated the Justice League), and had a weakness to wood instead of the color yellow.
Hal Jordan was the Green Lantern most people remember from the old Super Friends cartoon, Silver Age comics, and the Super Powers action figure from the '80s. He was the best GL there was, but sometimes he took time off.
That’s when Guy Gardner (a hothead with a bowl haircut reminiscent of Stooge Moe Howard) and John Stewart (a tough black architect) came in, as replacement GLs. I believe John became GL of Earth while Hal was out exploring space, and vice versa.
Then Guy lost his ring because he was a big jerk, and gained Sinestro’s yellow power ring, which had similar powers. John quit the GL Corps and joined an organization called the Darkstars at one point, and later lost his ability to walk. I’m not sure if this has been corrected or not.
Hal went nuts when his hometown of Coast City was destroyed by the alien Mongul during the “Reign of the Supermen” story, and became the villain Parallax during the “Emerald Twilight” story. He killed a lot of other Green Lanterns during his rampage.
That’s when the Guardians of the Universe (the beings that created the GL Corps) found Kyle Rayner and gave him the last GL power ring. He is the current Green Lantern in DC Comics, since about 1994.
Hal Jordan eventually redeemed himself by reigniting the sun during the “Final Night” story, sacrificing himself in the process. And he was finally brought back as the new Spectre, the spirit of redemption, during the “Day of Judgment” story.
Over the last few years, Alan Scott rejoined the JSA under the name Sentinel, Kyle joined the current JLA in the comics, and Guy became the weapon-morphing hero known as Warrior. Last I heard, Hal is still the Spectre.
John Stewart was selected by writers and producers to be the GL in the cartoon Justice League (a separate continuity from the comics), perhaps because they wanted a minority hero, but definitely because he had a strong and forceful personality which works well contrasted against the others. That is why you’ve seen merchandise lately with John as Green Lantern. He wasn’t a new “token black” character to insert into the team, but an actual important superhero who had been around since the 1970s.
