That is a fine question in need of a good answer. The answer for implementors is easily found in the spec. For the rest of us there is the exceedingly sparse entry (at the time of this writing) in Wikipedia.
That entry is somewhat pathetic at the moment. I mean who wrote that crap?! Hmmm.. Well taking a look at the history... I see that the perpetrator was...oh! me.
Well I am sure one of you can do a much better job than I did of elaborating on the topic.