(the Dawnguard banner)
Who are the Dawnguard? No, really. We know them as a faction revived by Isran (pictured below), a clan of Vampire Hunters in an age long past, their fort in ruin, but who were they? Bear with me as I attempt to tackle this question.
The Dawnguard trace their roots all the way back to the Second Era. The unnamed son of the unnamed Jarl of Riften was far too adventurous for his own good. Eventually, he came across a trio of vampires, and though he slew them, he contracted Sanguinare Vampiris. Fearing for his son if news got out, the Jarl of Riften formed the Dawnguard and charged them with keeping his son safe within the bowels of Fort Dawnguard, southeast of Riften, beyond the Dayspring Canyon.
(a map, in case you're feeling a bit lost)
Unfortunately for the Jarl's son - and the Dawnguard - he eventually regressed into the full stages of Vampirism and, since their duty was to keep him safe, the Dawnguard decided it would be best to kill him and end his suffering before he could take it out on innocent people. The Jarl of Riften saw this as an act of treason, and branded them criminals, but couldn't risk openly attacking them. The Dawnguard, meanwhile, decided to make it their mission to destroy Vampires. They were very good at it, their unique armor and weapons providing them the ultimate defense and offense against Vampires.
(The Dawnguard Armor - this variant Heavy - was designed to give a Vampire no exposed skin to possibly infect, while also providing extra magickal defense against their drain attacks. The Dawnguard War Axe was imbued with Dawnguard magic to enable it to deal increased damage to Vampires, by harnessing the power of sunlight. The Crossbow was designed specifically to kill Vampires before they got too close, with increased piercing power to penetrate the thicker skin of Vampires)
However, sometime after this, the Dawnguard became victims of their own success. They did so much damage to the Vampire clans, they found there were no stray Vampires left to hunt, the rest in hiding. The group is said to have disbanded and abandoned the Fort. Some rumors suggest that instead of wiping out Vampires, they, themselves, became Vampires, following in the footsteps of the Jarl's son, and preyed upon the innocent people of the Rift before someone else killed them.
(Crumbling Fort Dawnguard)
In 4e circa 201, Isran, a former Vigilant of Stendarr, recognizing the threat of the growing Vampire menace, decided to revive the Dawnguard, taking up refuge within the crumbling Fort Dawnguard. He gathered his followers - former associates and people who'd heard of his mission - within the Fort, bolstering his defenses with huskies and armored trolls (pictured below). Eventually, with the addition of an unnamed member, he had the force he needed to assault the Vampires. Leading a charge into the heart of the Vampire stronghold of the most voracious and dangerous Vampires in Skyrim, the Dawnguard attacked Castle Volkihar, seat of the Volkihar Clan. After a bloody battle, Lord Harkon, scourge of Skyrim, was defeated.
I wish there was more to tell about the Dawnguard, but that's all the information available to us. Neither Reddit nor the Imperial Library have anyone really talking about Dawnguard lore, so there isn't much to analyze. There simply isn't any more information available. Perhaps ESO might shed some light on the Dawnguard, considering ESO is set in the Second Era, but who knows? As always, all information is a direct result of UESP and the Imperial Library, as well as former Elder Scrolls Loremaster Michael Kirkbride.