The Dúnedain, or the Edain of the West, were the first Men who befriended Elves and their descendents who, at the beginning of the Second Age settled the Island of Númenor. When Númenor was destroyed in 3319 of the Second Age, only a few of the Dúnedain escaped, primarily Elendil, his sons, and a small group of followers who had not been corrupted by Sauron's presence in Númenor. There were also the Black Númenorians who settled in the Harads, and they were still servants of Sauron, though his body was destroyed. The Faithful Númenorians, those who had not fallen victim to Sauron's power, set up two kingdoms in Middle-earth, Gondor in the South and Arnor in the North.
The Dúnedain of the North were constantly attacked from 1300 of the Third Age forward, and thus were eventually forced to move from place to place until the Witch Realm of Angmar, which was the source of the attacks, was overthrown. The other Dúnedain dwindled also, until only those of the North survived with assistance from Elrond. All the males of the Drúnedain of the North went on to become Rangers, who protected the innocent Men and Hobbits of the North.
The Dúnedain of Gondor survived as well, though they were threatened often by the Black Númenorians of the West and the Harads in the South. The Dúnedain of this area had shorter lifespans than those of the Northern Dúnedain. Also, the line Kings in Gondor failed frequently. The bloodline of the Kings was also polluted by the blood of lesser Men. Some families, such as the line of Kings of Dol Amroth, however, had more noble blood, through marriages of Elves.
The Dúnedain were more noble then Men of other races, though they could fall into evil. They were tall Men, with dark hair and grey, noble eyes. Through the entire Third Age, the lifespans of the Dúnedain were shortened, though not to the levels of other Men. The Faithful Dúnedain were like to Elves in some respects, and they loved Elves. Elves like the Dúnedain as well. Some Dúnedain even posessed the power of foresight, something usually reserved to Elves.
The Dúnedain primarily spoke Westron, though it was of a fairer style than that of other Men, for it was incorporated with Elvish words, which many Drúnedain knew through their knownledge of Sindarin, and perhaps even Quenya, the two main forms of Elvish.