I know what the Journal of the Whills is, but I haven't found a lot of information as to their nature, such as where they came from, what they're capable of, or, most importantly, what they actually are. Are they deities, or are they the pinnacle of being connected to the Force, or are they the next (or final) step for those who become Force ghosts? Are they Force itself? I've oft-heard people refer to the "will of the Force," so that makes me curious if it's actually their will.
** I feel that it is important to mention that there is a common misconception that Yoda is a Whill. George Lucas has strongly denied this. Yoda's race is a mystery to fans, as far as I am aware, as Lucas has not revealed it yet.