The Green Lantern Corps are a group of interplanetary super heroes set within the DC Universe. They are overseen by The Guardians of the Universe and composed of 7200 Green Lanterns.
The Guardians of the Universe created the Green Lantern Corps around 3 billion years ago and since then they have essentially become the galaxy's police force. The Guardians realized their failure with the Manhunters was due to their creation's single-mindedness, lack of emotion (such as fear and courage), and most importantly the absence of morality and honor in the androids.
The Guardians chose to assign sentient beings that excelled in fearlessness and morality. These beings would then watch over their sector of the galaxy. The mission of the Green Lantern Corps is to protect the galaxy from interstellar threats of all kinds, from individual planets' problems to forces threatening entire systems.