When the hobbits first meet Aragorn, he appears to them in the Prancing Pony on the edge of Bree, as Strider. This is the name that he is known by in the wild, where he is a Dunedain Ranger, who has travelled the lands far and wide, encountered many enemies, and lived beyond the lifetime of normal men. But who are the Dunedain, this secret brotherhood to which the heir of Isildur belongs?
During Peter Jackson’s film adaptations of the Lord of the Rings trilogy, the audience gets the sense that Aragorn has been running away from his responsibility towards the throne of Gondor, fearing that he will fail in its rule as Isildur did many thousands of years ago. He fears that his bloodline is weak, and that there is no true strength left in men to be able to resist the power of the dark lord. So, in the movies, it is suggested that he joins the Dunedain as a way to escape his true calling as king, and instead spends many years of his life travelling and training with them.
This is not the case in the books, for in Tolkien's original works he is a member of the Dunedain because he wants to be worthy of the throne. He learns from them, tracks beside them, and develops himself with their many secrets, knowing that he will one day resume his rightful place in Gondor, and hoping that by being a part of the Dunedain, he will be honourable enough for this monumental challenge. He always carries about his person the shards of Narsil, the glowing sword that was broken when Isildur cut the ring from the hand of Sauron, as a reminder of his responsibility, and his need to live up to what the people of the white city need and deserve in their ruler.
Perhaps the disparity between the film version and the book version is because Aragorn was never originally meant to be the heir of Isildir in the early versions of the book. In fact, his name was Trotter (which is where Strider later derived from) and he was an unassuming hobbit, with no connection to the kingdoms of men.
It was only as Tolkien re-wrote that he realized the character was far more important, and was made for more than the simple arc he had planned. So Trotter was always meant to be a ranger, and it followed suit that Aragorn the man would be one too. And that is all that is ever said in the films, but in the books, readers meet several others of the Dunedain Rangers in the form of the Grey Company, a band of 30 who are led by Halbarad, Aragorns chief kin among them. The group of men also has 2 elves within their company, by the names of Elledan and Elrohir, who are actually Arwen’s brothers. They have been sent by Lord Elrond of Rivendell with a banner for Aragorn, a banner which further proves his heirdom.
But the Grey Company themselves come unbidden into Aragorn's service, turning up in Rohan without Aragorn being aware. They say they were summoned to some to his aid, but as he never sent the summons, it remains a mystery as to what magic was at work that called them to aid just at the very moment they were needed.
Legolas believes they were summoned by Galadriel, who can read the hearts of men, and must have known that Aragorn would need them when she conversed with him in the lands of Lothlorien. Together, the Grey company inform Aragorn that he must take a dangerous path, and that they will accompany him until his quest is complete. They must take the Paths of the Dead, and summon those ancient people into service, if they have any hope of winning the Battle at Pelennor Fields. So the group of 30 go beneath the mountains into the dark caverns, risking their lives to help their friend, and command the Dead to fulfill their long-ago oath and set themselves free by helping the heir of Gondor in his hour of need.
Together the Grey Company and the Army of the Dead defeat the Pirates of Corsair who sail in great ships from the South, take down the Oliphaunts of the Haradrim, and help the riders of Rohan secure the battlefield and drive out the hoard of orcs who are laying siege there. Halbarad falls on the batlle field, giving his life for Aragorn's, and it is a grievous loss for the whole of the company.
After the War of the Ring is won, it is unknown what happens to them, but it is assumed that they disappear back into the wild to keep watch over the lands, and protect the people from afar. Some of them, like the two elven brothers of Arwen, are believed to depart on the last great ships sailing to the Undying Lands, and are awarded a place of honor in the sacred west for all eternity.