From Destinypedia, the Destiny wiki
Guardians are Lightbearers who serve the Vanguard as protectors tasked with defending the Last City and Solar System against the forces of the Darkness, such as the Pyramids, Fallen, Hive, Vex, Cabal, Taken, and various other threats.
As Lightbearers, Guardians cannot recall their past lives and the deliberate pursuit of information on their past life is forbidden. The Guardians have defended the City for centuries, however, that defense cannot hold out forever. Guardians are charged with the protection of humanity's survivors, but many have splintered off into other groups with their own agendas. For example, it is commonplace for Guardians to align with one of the City's several Factions for support. Some, however, leave the City altogether, such as those aligned with the Concordat or the Cult of Osiris. Few have turned their back on the Light and humanity all-together, such as the villainous Dredgen Yor, the original wielder of Thorn.
Guardians are equipped with an advanced arsenal of weaponry and vehicles, as well as the ability to channel the Traveler's power (referred to by some as "magic"). It is said that some Guardians rarely speak.
Thanks to their Ghosts, Guardians are effectively immortal in most situations. They take advantage of this fact in a number of ways, whether its the live-fire training exercises of the Crucible and Lord Shaxx, or by making new discoveries via visions as Thanatonauts. However, Guardians can be permanently killed if their Light is drained by a powerful enemy, such as powerful figures of the Darkness or if their Ghost is destroyed. According to Emperor Calus, it is possible to permanently kill Guardians without having to drain their light first, but the method of how to do this has been forgotten by the Red Legion.
Guardians can also be killed permanently by destroying their Ghost first, removing their ability to resurrect, then killing the Guardian. 
The Guardians' origins are rooted in the very origins of the Last City itself, in the Dark Age that followed the Collapse. In the earliest days, when the survivors of the Collapse were gathering beneath the Traveler and groups like Takanome's Rangers were all that separated refugees elsewhere from certain death, the Guardians were known as The Risen. Having been granted the power to wield the Light by their Ghosts, The Risen lacked a common, unifying purpose, and took up roles as enforcers and bodyguards for the first Factions of what would become the Last City. As the Factions feuded over their competing ideologies and competed for influence among the residents of the fledgling city under the Traveler, some of The Risen became disillusioned and abandoned their Factions, seeking a higher purpose. With tensions between the Factions already near the boiling point, this triggered the Faction Wars, a violent conflict that only ended when The Risen ceased fighting each other in order to unite against a new enemy that threatened them all: the Fallen. In the end, the Fallen were repelled, the Factions' iron grip was broken, the city under the Traveler became the Last City with massive walls erected to protect it, and the Risen became known as the Guardians.
Centuries after the Collapse, a stray Ghost wanders through the ruins outside the Cosmodrome, searching for a new Guardian. It finds someone long deceased among the ancient remains and revives them. Ghost leads the new recruit out of the Cosmodrome and back to The Last City after finding a ship, and introduces them to the people of the Tower. The recruit returns to the Cosmodrome to explore; after slaying an Archon of the House of Devils, the recruit gains an audience with the Speaker, who confides in them that the Darkness is returning and that the recruit must help protect the City any way they can.
The recruit (now dubbed 'The Guardian') makes two important discoveries while in the Cosmodrome; the first is that the Hive, who had slain countless Guardians after they were beaten back to the Moon, have returned to Earth to prepare for an invasion. The second is that Rasputin, one of the legendary warminds, has survived the collapse, and has restored contact with the rest of the solar system. The Guardian travels to the Moon to investigate the Hive threat; they receive an urgent message that the Hive are attempting to drain the Traveler of its light and that they must be stopped at all costs. However, they also run into a mysterious stranger, who tells them to meet her on Venus to stop another rising threat.
After stopping the Hive ritual, The Guardian travels to Venus and discovers the Vex, a machine race bent on dominating reality itself. The Stranger tells The Guardian that the only way to stop the Vex and save the Traveler is to enter the Black Garden and destroy its heart. The Guardian seeks counsel with the Awoken of the Reef; Queen Mara Sov tells them that the only way into the Garden is to retrieve the eye of a Vex Gate Lord. They assume it to be a suicide mission, but The Guardian returns with the eye. The Queen tells The Guardian that they now owe her a favor in return for her information.
The entrance to the Black Garden, located on Mars, lies behind thick Cabal defenses. After successfully restoring the Gate Lord's eye using dormant Vex technology, The Guardian enters the Black Garden, a strange Vex realm outside of normal spacetime. At its center is the Black Heart, an abomination whom the Vex worship and seek to use to bend reality to their will. The Guardian kills its protectors, the Sol Progeny, destroying the Heart and freeing the Black Garden from Vex control. Light begins returning to the Traveler. But to finish off the Vex threat, The Guardian must enter the Vault of Glass, the source of the Vex's reality-altering powers, and kill its leader, Atheon.
The Dark Below
Eris Morn, the last survivor of a fireteam to stop the Hive god Crota, is looking for Guardians to help her. The Hive's return to Earth is but a prelude to the return of Crota, whom his followers are attempting to awaken. Eris sends The Guardian into the Cosmodrome to stop the invaders, especially after they attempt to kill Rasputin. She then sends The Guardian to stop the Hive's awakening ritual on the Moon. After eliminating Crota's generals, The Guardian finishes off Crota himself in his netherworld, the Oversoul Throne, deep within the Hellmouth.
House of Wolves
The Reef is embroiled in civil war as the House of Wolves, a Fallen house whom Queen Mara Sov had conquered, are now in open rebellion. Their leader, Skolas, has declared himself the Kell of Kells and wishes to unite the Fallen under his banner. The Queen has summoned the Guardians to aid her in ending the rebellion, in return for the Reef's treasures. The Guardian is aided by Petra Venj and Variks, The Loyal, with whom they search for Skolas and put a stop to his plans. After failing to seize control of the House of Devils and House of Kings, Skolas instead attempts to seize control of the Vault of Glass and the Vex technology within. The Guardian finally captures him atop the Citadel, stopping him from summoning his entire House through the Vex's time portals. Skolas is sent back to the Prison of Elders, where he's finished off once and for all.
The Taken King
Oryx, the father of Crota, has sent his Hive fleet to avenge his fallen son. Using his terrible power, Oryx creates a new army called the Taken, and lays siege to the entire solar system. The Awoken fleet attempt to stop him before he can enter the Inner System, but they are utterly destroyed. He then starts Taking the Cabal stationed on Phobos, which catches the Vanguard's attention. Responding to a distress signal, The Guardian is sent to investigate. Upon seeing the devastation, The Guardian is confronted by a vision of Oryx, who tells The Guardian that he is there to pay back humanity for his son's death. Realizing the threat that laid before them, Cayde-6 crafts a plan with The Guardian to sneak aboard Oryx's Dreadnaught. Though they are successful, they cannot enter Oryx's inner sanctum until The Guardian becomes Ascendant; Eris helps The Guardian recover the last fragments of Crota's Soul, allowing entry into Oryx's chambers, where they duel.
Though Oryx's physical form was defeated, he retreated even further into the ship, recovering his strength. Meanwhile, the Guardians work towards pushing the Taken out of the system. Once Oryx's lieutenants, the Court of Oryx, have been defeated the Guardians push deeper into the Dreadnaught to seek out Oryx within his Ascendant realm. There, he confronted the Guardians with his full might, even pulling them into a Darkness dimension. But in the end, using Light stolen from his pawns, Oryx was slain permanently.
Rise of Iron
The Fallen House of Devils, while looting ancient tombs from the Golden Age, accidentally unearth a self replicating nano-technology plague known as SIVA within the depths of the Old Cosmodrome. This plague mutates the Devils into powerful machine gods which collapsed The Wall in Old Russia, prompting Lord Saladin, the last of the Iron Lords, to personally lead the Guardians in a final stand against the plague, raising a new generation of Iron Lords to guard the Last City once more. The Guardian successfully ends the SIVA threat and is made an Iron Lord, with the title of 'Young Wolf'.
The Guardian ranks would be thrown into discord after the destruction of The Last City by Dominus Ghaul, commander of the Red Legion, one year after the SIVA Crisis and two years after the Taken War, after the Skyburner's distress signal reached Dominus Ghaul.
Guardian forces were left powerless after the Light had been sealed by the Traveler Cage, and were hunted en masse by Red Legion forces, with many being executed. Though trapped, the Traveler sent visions to the now lightless Guardians to seek out one of its fragments in the European Dead Zone, whereby they could regain their powers. Dominus Ghaul attempted and failed to win the Traveler's approval and make him into a Guardian, and simply took the light by force. When he gloated in front of the Traveler, the Traveler suddenly broke its cage and destroyed Ghaul, as well as slowly healing itself by drawing fragments back into itself. With the invasion ended and the city retaken, the Traveler was declared no longer dead but living once more, thus bringing back the light to the guardians all over again.
In a prison break at the Prison of Elders, Cayde-6 was permanently killed by Uldren Sov, resulting in decreased morale among the Guardians. The Vanguard ordered every available Guardian to track down and eliminate or capture every escapee from the Prison, but it still wasn't enough to prevent them from terrorizing the Solar system. One Guardian journeyed to the Reef and hunted down Cayde's killers, Uldren and the Scorn Barons.
After avenging Cayde, the Guardian discovered the Dreaming City, and Mara Sov ordered Petra Venj to allow Guardians into the city to combat the Taken corruption that was spreading through the city. This culminated in a Raid into the center of the Dreaming City by a fireteam of Guardians, where they successfully freed Kalli and Shuro Chi from being Taken, then killed Riven, of a Thousand Voices. However, in death, Riven granted one last wish to an entity Riven only referred to as "She".
There are three classes of Guardian: Hunters, Warlocks, and Titans. Hunters utilize swiftness, precision, and stealth to efficiently slay their enemies. Warlocks have the least amount of armor but the fastest regeneration and utilize a variety of projectile attacks in their abilities. Titans are able to both take and deal massive amounts of damage, prioritizing either one depending on their subclass.  Titans emphasize armor and raw power, while warlocks emphasize the use of their magical abilities and agility. Hunters fall in between both classes. The Guardians include members of the human, Awoken and Exo races, and can be either male or female.
While the choice of race is mostly aesthetic, the choice of class will have more impact. Note that all three classes have access to the same weapons, and are not "locked in" to any particular role. However, each class can specialize in a particular direction depending on which subclass the player is using. Choices of subclass will determine what Super the player can use, as well as what bonuses and other lesser abilities they'll have access to. Subclasses can be leveled up and switched on the fly, allowing a single character a degree of flexibility for different combat scenarios.
Guardians have three base stats determined by their choice of subclass: Armor (shield strength), Agility (movement speed and jump height), and Recovery (shield regeneration speed). Each class emphasizes each of these stats differently but can be modified slightly. Choice of armor can boost any of the three cooldown stats: Strength (melee cooldown), Discipline (grenade cooldown), and Intellect (Super cooldown).
While players gain levels with experience, these levels are mainly for determining what gear and missions are unlocked. The player's actual strength is determined by Light levels. Light represents the average attack and defense values of the player's currently equipped weapons and armor; every mission has a recommended light level, so players who meet that recommendation are more likely to succeed said mission. Generally speaking, items with more rarity have a higher light level than others.
List of appearances