The ranger class was introduced into OD&D not in an official supplement to the game, but in issue 2 of The Strategic Review, published in the summer of 1975. Written by Joe Fischer (who was, unless I am mistaken, a member of Gary's Greyhawk campaign), the ranger is obviously inspired by the character of Aragorn from The Lord of the Rings -- why else would the class be able to employ all items dealing with clairvoyance? -- and is described as "similar in many ways to the new sub-class Paladins." Thus, rangers have to be Lawful in alignment and remain so, may not employ hirelings, and are limited in how much they may own. The Strategic Review ranger is skilled in tracking, especially in the wilderness, and is a dedicated foe of "Giant Class (Kobolds-Giants)" monsters. He also gains the ability to cast clerical and magic-user spells, beginning at 8th level.
The impression I get from reading Fischer's original class -- and it's entirely an impression, since there's no expository text beyond game mechanics -- is that rangers are humans (and only humans, since this is OD&D) who learn the ways of the Wild in order to defend civilization against it. Like Tolkien's Dúnedain of the North, rangers are civilized men who forsake the comforts and safety of civilization in order to protect those they leave behind. To borrow a phrase, rangers exist in the Wild but they are not of it.
AD&D expands upon Fischer's original concept somewhat, but the basic idea behind the class remains the same. Though Gygaxian rangers can cast druid spells rather than cleric spells, as in The Strategic Review, they share nothing of the druidic worldview, which is described in the Players Handbook as "viewing good and evil, law and chaos, as balancing froces of nature which are nbecessary for the continuation of all things." AD&D rangers, however, must be good in alignment and any change to a non-good alignment (even a neutral one) strips the character of his unique ranger abilities.
For some reason, though, it's common for many gamers to think rangers are somehow connected to druids and their nature religion, when, from my perspective, rangers and druids would most likely be foes, or at least often at loggerheads. Rangers are strongly aligned with Law and thus civilization. That's certainly how I've always understood and portrayed the class. That's why I don't think non-humans, especially elves, should be permitted to become rangers: the class is all about taming the wilderness or at least keeping its worst elements in check for the betterment of mankind. They're not nature's "champions" so much as individuals who've become intimately familiar with nature in order to better combat its darker side.
I'm currently working on a version of the ranger for use with Swords & Wizardry that draws a lot of inspiration from Joe Fischer's original. I'll probably post it here when I'm done with it.