The Guardians of the Universe are an alien race. They are the founders of the Green Lantern Corps.
Not much is known about the Guardians besides the fact that in their first attempt to enforce their will and guard against all menaces, about 3.5 billion years ago they created a legion of robotic sentinels called the Manhunters. However one of the Guardians was possessed by a cosmic entity known as Parallax and then re-purposed the Manhunters as weapons of fear. So as a response the Corps was created to combat the threat. In order to battle the Manhunters, the Guardians took extreme measures, as the rings powered by the emerald energy of will were not enough so they created the yellow rings, that are powered by fear. Unfortunately, the yellow light also drew something from the darkness, Parallax. During the war planet Oa was severely damaged and now all the Guardians are considered to be dead.
Traveling to an unknown part of the galaxy, John Stewart and Superman come face to face with Parallax. The two Lanterns see that Parallax has imprisoned the presumed dead Guardians and feeds from them. Parallax says that they offered themselves to him as part of a trade, a treaty, their fear as eternal fuel for his hunger for the power they siphoned from his domain with the promise of never hunting him again. Clark tells Parallax that the terms of the treaty haven't been violated but Parallax says the Lanterns are gathering to destroy it, with Clark being the newest member of the Corps as proof of it. Threatening Clark, Parallax says that the Guardians have lost their luster and the Manhunters have failed so he seeks to feed from the population of Earth.
Parallax has captured Clark and John and says that his yellow rings have arrived on Earth, by now, and his "harvesters" will provide him with the fear of the humans, so he can feast on it. One of the imprisoned Guardians, Ganthet, communicates telepathically with the two Lanterns and informs them that the other Guardians are keeping Parallax distracted, so he can tell them that in order to defeat Parallax, they should find something the beast fears. He has more to say, but he's unfortunately cut off by Parallax, who promptly tears him in two. Clark says that Ganthet bought them some time, by distracting Parallax, and they need to fall back.
During Stewart's battle with Parallax, the entity uses a manifestation of the Guardians to make him give in to fear. Moments before overtaking him, Parallax tells Stewart he's been deceived by the Guardians, that he should let all of that go and embrace his fear.
With the original Guardians being killed by Parallax, the remaining Green Lanterns have decided to form the New Guardians, with one of them being John Stewart. The New Guardians having learned from their predecessors' mistakes, decided that from now on no one else will ever get conscripted into the service against their will and that they should not fear change.
In the Comics
The Guardians evolved on the planet Maltus, and were among the first intelligent life forms in the universe. At this time they were tall greyish blue humanoids with black hair, who roughly resembled humans except for their skin color. In a pivotal moment, billions of years ago, a Maltusian named Krona used time-bending technology to observe the beginning of the Universe. However, this experiment, and later attempts to stop it, unleashed disaster upon all existence. Feeling responsible for this, the evolved Maltusians relocated to the planet Oa (at "the center of the Universe") and became the Guardians. Their goal was simple: combat evil and create an orderly universe. They acted quickly on that goal. During this period they slowly evolved into their current appearance, losing about half of their height and having their skulls grow larger. They now act as the leaders of the Green Lantern Corps, an interstellar police force which patrols the universe.
The Guardians were almost wiped out in the events of Emerald Twilight, the only survivor being Ganthet. They sacrificed themselves to create one final power ring, a power ring perhaps more powerful than all others before it. Oa was itself destroyed in a battle between Parallax and Kyle Rayner, but rebuilt in the events of Legacy as the final wish of Hal Jordan's former power ring.
The Guardians have since been restored when Kyle Rayner, as Ion, recharged the Central Power Battery.
During the War of the Green Lanterns storyline, the Guardians have been discussing the various events occurring ranging from Hal Jordan's allegiance with the New Guardians, the revolt of the Alpha Lanterns etc. The group is then later confronted by Krona who reveals his plan to control emotions. As a result he had the Butcher, Ophidian, Ion, Adara, Proselyte, and the Predator inhabit the six Guardians as hosts and places Parallax back within the Green Central Power Battery to subjugate the Green Lantern Corps under his control. The Guardians are freed from Krona's control emotions by Hal Jordan, who kills Krona using his ultimate power. The rings from the other corps return to their former wielders. However, the Guardians expel them from Oa.
In the Rise of the Third Army, believing the Green Lantern Corps is ultimately a failure, due to the members emotions becoming an unpredictable all the Guardians believe it is time to replace the Corps, except for Ganthet, who is later indoctrinated by his brethren. Before the begin of their plan, The Guardians use Black Hand to seemingly kill both Sinestro and Hal Jordan, an act which would lead to Simon Baz receiving Sinestro's ring. The Guardians then use the power of The First Lantern Volthoom, to create the The Third Army, an army of mindless creatures who take over and transform their victims to multiply their numbers. All of the various Corps learn of the Guardians plan and begin to combat the Third Army, and though the Third Army would be defeated, Volthoom, the First Lantern would become free, and seek revenge on the universe. After defeating Volthoom, Sinestro seaks his revenge on the Guardians, slaughtering every single one. Though the Green Lantern Corps believes the Guardians to be dead, secretly, Sinestro spared the life of Ganthet, reuniting him with his love, Sayd.