The Green Lantern Corps is an Intergalactic Military/Police Force made up of several thousands of alien Species. They patrol the farthest reaches of the Main Universe, and come from the DC Universe, at the Behest of the Guardians, a race of Immortals residing on the Planet Oa. Not much else, however, is known about them.
Oa is the Planet that all Green Lanterns come for Training, and all are called there in times of extraordinary crisis. This is the Planet that the Guardians came from, and also the System in which the Core of the Green Lantern's power is hidden.The Planet at first appears Descolite and Lifeless, the Surface being composed of Black, Craggy and Spiked Rock. However, closer inspection shows large Futuristic structures of an Alien Metal, as if there were several spots on the Planet that were cities. Outside the largest city is where the Lanters gather in times of crisis; out in the barren wastelands that make up most of the planet.
It seems that the Planet's inner structure somehow houses massive stores of energy, and the very world itself may seem, to more powerful beings, to be alive itself.
It is not known how the Lanterns patrol the vast Universe on their own; The Exact number of Green Lanterns serving the corps is also unknown. Each Green Lantern is given a Power Ring, a weapon granting the use of incredible abilities that are directed by the wearer's own willpower.
Sinestro is a Redish-skinned humanoid Alien; He is the Leader of the Green Lantern Corps. He takes his duty of preserving Justice throughout the Universe very seriously. Normally, he observes the missions the other Lanterns undertake on Oa; however, when it is required, he himself will go out to face Danger, along with a group of other Lanterns. He is very serious, very straight-forward, hard to trick, hard to persuade (Unless it has to do with preserving Justice) and nearly unable to defeat with Physical, psychic, Magic, or persuasive power.
The Power Rings issued to the Green Lanterns are amazingly Powerful Constructs. It appears that the Lanterns get their power through the ring by their own Willpower; meaning, anything they can Imagine, the Ring will create (In the form of energy the Corps call "Solid Light Constructs"). The only actual limit to a Lantern's power is the limit their minds have to think.
Additionally, The Rings carry a limited Charge; when their energy is used up, it is possible to recharge the Ring by joining it with the Personal Power Battery given to Each member (Which looks like an old-fashioned Lantern), and saying the Oath of the Corps.
Currently, other weaknesses include the inability to affect things colored Yellow, and the inability to kill sentient Beings. Why these inhibitors exist is yet to be determined.
"In Brightest Day, in Blackest Night,
"No Evil shall escape My Sight...
"Let those who worship Evil's Might,
"Beware my Power; Green Lantern's Light!"
It has been known that the corps use overly excessive forces against others. Its also known that they try to be in every country and organizations affairs even when it does not involve them.
Whatever the case, it would be best to ask about Rumors, seeking answers from a Lantern. It may be true they use exessive force, though this is viewed by opinion; Their involvement is also a possible questioning factor. The only way we'll know more is if we bring these questions to them.