Who is he? Just some guy who brought us BSD, NFS, the UltraSPARC and parts of Java. To the uninitiated, these terminologies probably don't mean much to you, possibly other than Java (Yes, they are the guys who trademarked the name of an island in Indonesia because they couldn't screw around the laws of their own country to name it after a tree).
Besides being the creator of these obscured things, his important contribution to Computer Science to me, is vi, the precursor to the vim editor that I use almost on a daily basis.
Vi is a very old piece of software that was created in 1976. Even as a text editor older than I am, it one of the most remarkable software that I've used. Not that it is the most earth shattering piece of software, but that some people like to argue that Google Earth is more rightly so, yeah right, you laymen. Pfffffffft.
While I find vi archaic enough, vi was born out of Joy's frustrations with another archaic editor called 'ed'. Just to highlight how l33t these guys are, they had to name everything in two archaic letters, just like the 'holy trinity' of Unix commands 'ls', 'cp' and 'mv'.
More talented than the other Bill (That email@example.com, who was said to have written BASIC over the course of one weekend), Joy known for writing vi over the course of a single weekend too, as well as writing the entire TCP/IP stack of Berkeley Unix by himself.
And he only did so because he thought the TCP/IP stack written by contractors hired by DARPA was inferior. And when asked how he managed to do it, his smug response was, 'It's very simple -- you read the protocol and write the code.'
(Before I get a lawsuit from either 'Bills', I better clarify that most of it probably isn't true. Bill Joy wrote vi in months rather than a weekend, and wrote parts of TCP/IP as opposed to the whole thing. And he probably didn't say that either.)
Just like a smug geek of his days, he's has done the world of Computer Science a whole deal of good by creating a great deal of useful software, unlike today's smug little shit, who is only good at creating useless things that all my friends seem so addicted to. :P