Who was he, how did the legend develop, and how much of it is based in reality.
My area of expertise is primarily medieval Archery rather than ballads/songs but there is some overlap in this specific area so I'll do my best to provide an answer. Much of my reading on Robin Hood is from Jim Bradbury's book The Medieval Archer (which I recommend to anyone interested in that subject) and a handful of articles (although those I remember less well)
Whether Robin Hood really existed or not is pretty much inconclusive. There's no rock solid evidence of an original Robin Hood but neither is there necessarily solid proof he never existed. One thing worth noting is that early mentions of Robin Hood place him around Barnsdale rather than Sherwood.
The actual evidence for Robin Hood comes to us through ballads. The oldest ballad survives from the rather late date of 1450 and the most famous (the lytell geste of Robyn Hode) survives in five versions collectively from 1500-1550. The stories are no doubt older than these dates suggest, these are just the oldest copies that survive, but it can be hard to say exactly how old.
As to whether or not they are based in reality that's really a two part question. Are the events of Robin Hood (especially as we now know it) historically accurate? No, not really. While it's unlikely that John and Richard I got along all that well (none of their family seemed to) this idea of John trying to take the throne and then failing because of an outlaw is clearly a stretch. It's especially odd when you consider that historically Richard died only a few years after returning from Crusade and John ruled as king for twenty years.
As a reflection of its time, however, it's not so bad. You should probably consider it a reflection of later medieval England (1300-1500) rather than the alleged date of its events (notably early versions of the ballad don't actually give it a specific historical setting). The popularity and story of Robin Hood celebrate the Yeoman Archer in a time when English Archers were a major part of an English army that was probably the dominant force in Western Europe. It's also worth noting that outlaws hiding in forests were no rarity in England although I'm sure few would make heroes of them.
If you want to know more J.C. Holt's book Robin Hood (London, 1982) should answer most of your questions.
hi! you may also be interested in responses to these previous related questions
Who was the real Robin Hood and when did he live?
What is your opinion on whether King Arthur and Robin hood are real?