Oh fuck yes! It's Morgan's time to shine!
Destiny Lore 101: THERE IS NONE
In all seriousness though, it's actually pretty complex and detailed as hell. I'll bring you up to speed.
In the beginning, there was man. Man had always aspired to ascend beyond Earth, to find it's Destiny among the stars. It was on one fateful Mars mission that a group of three scientists discovered the Traveler. We did not know the full capacity of this giant orb of awesome, except that it could grant us a treasure trove of knowledge and technology.
This ushered in a time we Humans knew as 'The Golden Age'. It was a time of flourishing scientific discovery, weapon advancements, and space travel. Not only did Humanity finally bring it's dream to populate the stars a reality, we were able to discover cures for unthinkable illnesses, and Human life span tripled as a result.
However, with the wonders that the Golden Age brought, nothing could prepare us for what would come. The Darkness, the Traveler's ancient enemy, had finally found our savior. Humanity fought, alongside the Traveler and their newly created Exo Humans, against the Darkness... but it was all in vain. The Darkness, while suppressed by the Traveler's dying breath, was successful in tearing Humanity off of it's self appointed throne, throwing us into a time known only as 'The Collapse'.
Where the Golden Age brought peace, knowledge, and life, the Collapse brought terror, removed our knowledge, and death. It was a dark time for Humanity. Many, like the Dead Orbit faction, looked to the stars once more, leaving Earth for the Darkness, hoping to start anew elsewhere. These people, influenced by the Darkness, would eventually become a sub species of Humans known as the Awoken, a Matriarchal society that neither fought for the Light of the Traveler, or the Dark of the Darkness.
Other groups, such as the Future War Cult and New Monarchy, thought the collapse as a time of new beginnings. FWC wanted to use the Collapse to bring Humanity's militarized ways forward, ushering in a new Militaristic Government, while the New Monarchy firmly believed the Traveler was still alive, and that the Speaker, the one to communicate directly with the Traveler, would become King.
There was one group, however, that only saw the Collapse for what it was; devastation for Humanity. The Vanguard wouldn't let the Collapse take Humanity down, they'd fight to win, or die trying. When the Traveler 'died', it created these small orbs of lights known as Ghosts. Ghosts are a Guardian's life-line, as long as you have your Ghost intact, you can die and be revived anew.
Using it's freshly formed Ghosts, the Traveler sought out Humans destined for greatness, regardless of when they died, to become Guardians. Reborn from the light of the Traveler, Guardians are able to use all forms of Light as a weapon. From there, three separate 'Guilds' were formed; Titan, Warlock, Hunter.
Titans stood at the head of the Vanguard, they were the direct fighters of the three classes of Guardians. They are excellent Tacticians and can easily soak up hits for other Guardians. Wielding Arc and Void Light, Titans can create neigh impenetrable spheres of Void Light to protect allies from harm, and even improve their allies and their own abilities as a result. Likewise, they can use Arc Light to create a pulsing circle of electrical light around themselves and damage foes with the Fist of Havoc.
Warlocks are the Intelligence and Support class of the Guardians. Warlocks, while still direct, tend to hang back in combat to hit foes at a medium distance. They take information from the Hunters, analyze it, and bring it to the Titans to assist them in combat. Armed with Solar and Void Light, Warlocks can use the Void to tear their enemies apart at a molecular level with their Nova Bomb. But if their allies are in a pinch and they find themselves dead, they can pull themselves from the Void of Death using Solar Light, reviving themselves without the need of their Ghosts.
Hunters are the Scouts of the Guardians. Hunters tend to stay out of combat in a direct sense, they prefer to stay farther back than the Warlocks and Titans, but have the massive advantage of turning the odds of battle in their allies favor. Preferably using Long-Mid range weaponry, the Hunters are survivalists. Just their armor alone allows them to survive in the wilderness for weeks on end. They tend to gather information to bring to the Warlocks. They use Solar and Arc Light to fuel their abilities, whether it is to take that perfect shot with the Solar enhanced Hand Cannon- or the grace of an electrically charged knife/sword.
Together, the Hunters, Warlocks, and Titans form the Vanguard. With only the Speaker to communicate to them through the Traveler, these Guardians go where no other man or woman dares to go. They go beyond the only Safe City left, finding resources and solutions, their ultimate goal; destroy the Darkness, return Humanity to it's Golden Age.
AAAAAAAAAAND there you have it. At some point I'll right a small synopsis on the enemy arch-types if you want. Give a little data on weapons and the enemy factions.