Long ago, the Guardians Of The Universe decided that they would be the protectors of law and order in the universe. Their first attempt at a galactic police force, the Manhunters, was a failure, due to their lack of sentient emotion.
The Guardians turned to sentient beings from across the universe, and thus created the Green Lantern Corps. The Guardians forged rings, which channeled pure willpower from their users into solid constructs and objects. The rings were powered by the Central Power Battery, another object the Guardians created.
Alan Scott's Ring
The Guardians of the Universe gathered all random magic and fused it into the Starheart, which gained sentience. A piece of the Starheart fell to Earth. It was fashioned into a rail road lantern which, during a train disaster, saved Alan Scott of Earth. Creating a ring from part of the lantern, Scott fought crime as the "Green Lantern."
Alan Scott's power ring has many capabilities. It creates anything the wearer can imagine and it gives the wearer the ability to fly, but it's magic cannot affect wood and things related to wood, like cellulose particles. The ring must be recharged every 24 hours by touching the Green Lantern for 10 seconds.
Original Corps Rings
Deciding to form a galactic peacekeeping corps, the Guardians forged a giant, main Central Power Battery on planet Oa, combining their own energy with the planet's ambient power. They chose 3600 agents and gave each a ring, a personal power battery linked to the Central Power Battery, and a sector of space to protect. A Corps member could select his own successor, as Abin Sur did with his test pilot Hal Jordan.
These rings, just as Alan Scott 's, create/do anything the wearer can imagine, but this time their powers included time travel, and they also allow the wearer to fly. The ring's potency depends on the willpower of the wearer. The rings can also create duplicates of themselves. Reserve power automatically protects the wearer from mortal harm. The ring must be charged every 24 hours by touching the battery for 10 seconds, during which time the Green Lantern will usually recite their oath, and it's energy cannot affect anything colored yellow.
Oan Power Rings
In the days before the original Green Lantern Corps, the Guardian known as Meadlux created a power ring specifically for the Guardians. An Oan power ring possessed the same capabilities as a standard ring, but was fueled by the internal power of its wielder and thus did not need to be recharged. Unlike the traditional Green Lantern power ring, an Oan power ring did not contain the impurity that restricted its capabilities against materials colored yellow. The revelation that such a weapon existed caused a great schism within the ranks of the Green Lantern Corps. Officers who had lost close comrades due to adversaries empowered by yellow weaponry lost faith in the Guardians, some of whom, such as Galius Zed, Eddore, and Kaylark even went so far as to turn their backs on the Guardians, coveting the rings for themselves. Another group of individuals who took an interest in the Oan power rings were the space pirates known as the Free Lancers.
Power Ring's Power Ring
Power Ring, a member of the Crime Syndicate of America, wielded a Power Ring that utilized mystic energy, enabling him to fight and sometimes overcome even the Crime Syndicate of America's counterpart to Superman, Ultraman. Its powers were the same as Hal's, but without the yellow weakness, or Alan's wood weakness.
Kyle Rayner's Ring
After Green Lantern Hal Jordan went rogue - destroying the Corps, the Guardians, and the Main Battery - the sole remaining Guardian, Ganthet, gathered the fragments of Hal's shattered ring. He created from them the last Power Ring and conferred it upon a young Earth man, Kyle Rayner. Kyle's battery is a piece of the original Main Power Battery.
Just as the rings before him, Kyle Rayner's ring creates anything the wearer can imagine and allows the wearer to fly. This special ring though is keyed to Kyle Rayner's DNA. It can only be used by him, those related to him, and Hal Jordan. The power supply is expended according to what the ring creates rather than the length of time since it's last recharge. The ring must be recharged by touching to the battery for a period proportionately equal to how "empty" it is. It gives the wearer a sense of "full" or "empty." Rayner's power ring is invulnerable to every known substance.
Sojourner Mullein's Ring
Jo's ring does everything a regular Green Lantern ring does, however, it does not need a power battery to recharge. Instead, it recharges incrementally over time. A side effect is if it ever depletes completely, it takes a lot longer for it to be useful, leaving the ringbearer vulnerable for longer. It has also been specifically designed to not power itself based on the will to overcome fear but rather the will to live with it. According to the Guardian that gave it to her, this causes more random spikes in sheer power. To save on power, she has an implant with a translator program called "Meatware" so she can understand the people of The City Enduring instead of using the ring's universal translator.
New Corps Rings
The new, reformed Corps has new rings. With the destruction of the cosmic entity of fear, Parallax, came a new age for the Green Lantern Corps. Kyle Rayner single-handedly reformed the corps.
Also with the destruction of Parallax came the removal of the "yellow impurity," as it is now called, and only new Green Lanterns have trouble with the color yellow. They must feel fear, and overcome it, before they can effect the color yellow. Other than this, the new rings are almost exactly like the original Corps' rings.
Other Power Rings
The Book of Oa has a forbidden chapter on cosmic revelations, which included prophecies about the Blackest Night. At the end of the Sinestro Corps War, the Guardians of the Universe known as Ganthet and Sayd reveal to the four Green Lanterns based on Earth the final verse of the prophecy, known as the "Blackest Night", which reveals that five more Corps, one for each of the remaining colors of the rainbow will be created in the future and that war will break out between the seven Corps until they destroy each other and the universe, leading to the Blackest Night prophesy.
- Yellow Unlike the Green Rings of the Corps which operate by channeling pure willpower, these rings which are forged on Qward channel pure fear. Several versions of these rings have been created, but recently they have become the primary weapon of the Sinestro Corps, seeking out individuals to battle the Green Lantern Corps who have the ability to instill great fear.
- Blue Blue Power Rings are shown to symbolize hope, and were forged by Ganthet and Sayd, two former guardians. The Blue Lantern Corps' rings only work when an active green lantern is present.
- Red Red rings channel rage and hate. They appear to manifest their power out of the wielder's mouth. It hasn't been confirmed yet though.
- Violet Violet rings, forged by the Zamarons, are shown to embody love.
- Indigo Indigo rings are fueled by compassion, and its users are ambassadors of goodwill and are much like shamans
- Orange Orange power rings are based off of avarice, also known as greed.
- Black A massive black lantern surrounds the Anti-Monitor's body as he lands on an alien planet. It is revealed to make a black power ring, which latches on to the dead. They will bring the Guardians' prophesied "Blackest Night."
- White The white power ring attached itself most notably to Boston Brand at the end of Blackest Night after resurrecting him and the other previously dead heroes.
No hard upper limit to the power ring's capabilities has yet been demonstrated; it is often referred to as "the most powerful weapon in the universe."
The power ring's most distinctive effect is the generation of green, solid-light constructs, the precise physical nature of which has never been specified. The size, complexity, and strength of these constructs is limited only by the ring-bearer's willpower; whatever the wearer imagines, the ring will create.
When active, a power ring will encase its user in a protective, life-supporting force field. This force field allows the user to fly, travel through inhospitable environments (outer space, underwater, etc.), and enter hyperspace in order to move vast distances quickly. The ring also generates its wearer's Green Lantern uniform: the uniform appears over their normal attire and vanishes at the user's will.The uniform varies from Lantern to Lantern, based on anatomy, personal preference, and the social norms of their race. The only rule in this regard seems to be that the uniform must openly display the symbol of the corps, though even this has been modified based on preference, (a vampire-hunting Lantern adapts the symbol into a cross, and a blind Lantern with no concept of light or color uses the image of a bell) as in the case of Kyle Rayner who wears a modified version of the symbol on his uniform.
Power rings also appear to be highly advanced computers; they are able to talk to and advise the wearer as to various courses of action, as well as act as a universal translator. The ring can also scan for energy signatures or particular objects. For more intricate problems or problems that require a back logged history (Of a planet, person, group, etc), the ring connects with the main power battery on Oa which is the "main" computer, of sorts.
Power rings are able to give off electromagnetic radiation of various frequencies. This radiation can be focused by the wearer into a beam, similar in appearance and effect to a powerful laser. The ring is also capable of producing an electrical current. Less frequently used capabilities include splitting atomic nuclei and manipulating subatomic particles (thereby transmuting chemical elements). A power ring is also capable of creating fully functional duplicates of itself.
While power rings have to be worn to be effective, at several points Green Lanterns have shown the ability to summon the ring to them from a distance (even if someone else is wearing it) or order it to carry out commands automatically after being removed. Some power rings have been shown to be genetically keyed to the wearer, like Kyle Rayner's, though villains have circumvented this through various means. For instance, Manhunters use tissue samples to make Kyle's ring think it is still on his hand. When a Green Lantern is slain, their ring will automatically seek out a suitable replacement after logging the death for record. While the Green Lantern ring is an extremely advanced computer and is highly intelligent, it cannot think for itself. The ring will only act if it is given instructions by its wearer. Also, when a wearer expires, it will seek out a new replacement, but at that point, Mogo, the planet sized Green Lantern takes over telepathic "custody" of the ring and issues its commands helping it find a suitable new recruit. Without Mogo, the ring would just transport itself to Oa and fall to the ground inert until it is needed again.
The Ring is a source of limitless power, and thus there is no limit to what it can do, but here are some abilities it has been shown to have.
Every ring has a connection to the Main Battery on Oa, which taps into an artificial intelligence. It acts as an "on-board computer," telling the wearer what they need to know. It can either respond out loud, or silently directly to the wearer's mind. The AI contains a large database of information that may be crucial to a Lantern's success. The ring also translates nearly every language to and from the wearer (though they have difficulty translating profanity), which is why the Corps can communicate with each other. When the bearer of a Green Lantern ring dies, the ring will seek out a suitable replacement for their sector. The AI can be used to play a holographic playback sequence complete with colors across the spectrum based on information in memory banks. The ring can dictate when to pause or stop the playback should an interruption arise where the Lanterns undivided attention is needed. The AI can also alert the wielder of incoming threats or of attempts to manipulate the wielders construct by an outside party.
The ring is programmed with every language in the universe, and translates all alien languages into the language thee wielder speaks. This helps eliminate the language barrier and facilitate communication between lanterns.
Every lantern is protected by an invisible force field that eliminates waste, provides a breathable atmosphere for the wielder in any environment(underwater, space, etc.) and protects the wielder from all forms of harm(radiation, bullets, etc.).
Via manipulation of gravitons, lanterns can hover or fly in any direction. In space, they can fly at faster than light speeds.
Rings create wormholes to allow lanterns to travel between the stars in a short amount of time.
A lantern can create anything he or she imagines, assuming they have enough willpower to do it. The objects created by a green lantern are not indestructible, and the strength of the constructs depends on the focus and willpower put forth by the wielder. If the wearer's willpower is weak, then their constructs will be weak as well. Depending on the amount of focus and willpower from the wearer, the ring can be the wearer's greatest asset or greatest detriment.
Each ring can duplicate itself, creating a second ring which may be given to another as a backup, for protection, or to help the lantern in times of great need. This duplicate ring is exactly like a normal ring.
Besides light based radiation used to create the energy constructs associated with a Green Lantern, the ring can simulate various forms of radiation. One example of this is the ability to simulate the radiation of Green Kryptonite, a form of radiation that is harmful to Superman. However, this ability is no longer valid and has been retconned.
The ring allows the wielder the ability to travel through time, though the process requires great willpower.
The ring cannot be wielded correctly if the wearer is under the influence of drugs or if there is an involvement of neural interference.
In the beginning the Green Ring was vulnerable to the color yellow. It was unable to defend against attacks from wielders of the Sinestro Corps as well. This weakness however has now be changed and can be overcome by veterans who have the ability to overcome great fear.
Previously had to be recharged every 24 hours or when it has been depleted of energy. This can be accomplished if the bearer of the ring recites an oath into his power lantern while holding the ring to it. However now, the power ring only needs to be recharged when it runs out of energy.
Ability To Kill
One of the main weaknesses of the Power Rings that greatly disturbed Sinestro was the fact that the bearers of the Green Lantern Ring were unable to kill. Recently however, this restriction on the rings has been lifted but murder is still against the law of the GL Corps.