The Guardians of the Universe are the last of an ancient race in the DC Universe who tasked themselves with defending the universe and created the Green Lantern Corps. The Writers switch between saying that Guardians are able to do everything that green lanterns do or that they also need rings to use that emotional spectrum energy.
In the cartoon Superman, the guardians met the green lantern teleporting him to Oa (they appeared in some episodes of the sequel: Justice League). In New Frontier they talked to him by the ring. In the film with actors the scene was of him being taken to the planet by the ring to speak to them. In First Flight they couldn't do any of that, just send other flashlights to bring him to Oa to talk to them. In Batman The Brave and the Bold appeared at the end of an episode, participated more in the episode about Reach. In the video game Teeny Titans, in the moments when Oa is the scene of the fight statues of the guardians are seen. In one of the introductions to the video game Injustice, before the fight Batman asks why he should trust Hal Jordan, he says that the guardians trust and Batman replies that they also trusted Sinestro.