One theory about Robin's origins that I've come across in the past is that he's basically a humanized version of Robin Goodfellow, better known as Puck. Yes, this theory would mean he was originally a fairy, which would explain his clothes.
It seems that this idea isn't too popular among modern scholars, however, with the more likely explanation being that Robin was always human. It's entirely possible that "Robin Hood" was simply a slang term for an outlaw, with references as far back as the thirteenth century referring to criminals as "Robinhood" and variations like "Robehod" and "Hobbehod." (English spelling had not been standardized at that point, so everything was spelled in several different ways.) But did the makers of the legends simply use a common term for "outlaw" as their hero's name, or was the term based on an actual man with the name? "Robin" was typically short for "Robert," and there were many people in those times named Robert Hood or Robert Wood. Even if there really had been a bandit chief named Robert Hood, however, did he really bear any relation to the figure we've all come to know and possibly love?
Many aspects of the Robin Hood legend are still typical in modern interpretations date back to the earliest known songs and stories of the robber. Even in early ballads, his home was said to have been Sherwood Forest in Nottinghamshire. The earliest extant texts date from the late fifteenth and early sixteenth centuries, and these refer to Robin's companions as including Little John, Will Scarlet, and Much the Miller's Son; as well as to the Sheriff of Nottingham as his enemy.
What isn't there is the historical context in which Robin usually appears in today's tales. I'm sure we all know the story of Robin being a supporter of Richard Coeur de Leon, who turned rogue during the time when Richard was away in the Crusades and his brother John was serving as regent. The early ballads, however, refer to a King Edward as being on the English throne, which was the case between 1272 and 1377. Richard, of course, ruled considerably earlier, and it wasn't until the sixteenth century that the legends came to be specifically set in the late twelfth century. It was also in the sixteenth century that Robin came to be regarded as an aristocrat rather than a yeoman, as he had been in the ballads. Anthony Munday's plays, which he wrote at the tail end of the sixteenth century, made Robin the displaced Earl of Huntingdon. He may have been inspired by the actual Earl of Huntingdon during Richard's reign, who was named David rather than Robin or Robert, but was known to be a supporter of Richard and opponent of John. His fortunes rose and fell with the political posturing of various rulers, and there's a tradition of his losing some of his land due to debts.
Other popular parts of the Robin Hood legend developed over time. One is his love interest Maid Marian, who didn't appear in the earliest ballads. It's not entirely clear how she first came to enter the picture, but some link her to David of Huntingdon's wife Maud or Matilda, sister of the Earl of Chester. There also appears to be a competing tradition that Robin and Marian were unrelated parts of the May Day celebrations at first, but later came to be associated with each other.
It wasn't until the seventeenth century or so that Robin came to be viewed as a patron of the poor, and it was also around this time that the minstrel Alan-a-Dale came to be known as one of Robin's companions.
I'm not entirely sure when and where the idea of Robin and his Merry Men being expert archers originated. While I believe the longbow did exist in the time of Richard and John, it wasn't in common usage in England until some time later. The earliest known printed tale of Robin says that he and his band use swords as their main weapons.
Okay, I think that's enough for now. If you want to know more about Robin Hood, consult your local library. Or Wikipedia, which has a pretty thorough entry on the bandit. This was another interesting Robin Hood page I found through a quick Google search, and it contains information on several historical personages who might have influenced the popular outlaw.