As a regulated telecommunications monopoly, AT&T was not permitted to make money selling software, so they couldn't offer Unix as a product. They could give it away in unsupported source form* to accredited educational institutions. One institution that got it around Unix Version 6 was the University of California at Berkeley. Bill Joy was then a graduate student in computer science, and a member of the Computer Research Group. He became the chief architect of CRG's efforts to extend and enhance the AT&T code. The result was BSD (Berkeley Standard Distribution) Unix, which became popular in scientific and educational installations. Because it contained copyrighted AT&T code, you had to have an AT&T source license to run it.
In the process of development, Bill wrote the C shell, a command interpreter with a C like script language and the vi editor among other contributions.
After leaving UCB, Bill became co-founder and VP of R&D at Sun Microsystems, which unsurprisingly chose BSD Unix as the flavor that was originally issued on their machines, and was one of the architects that developed the SPARC (Scalable Processor ARChitecture) CPU used by Sun machines. He was also the chief architect of Sun's Solaris OS, as part of an effort by AT&T and sun to produce a converged OS that would unite the AT&T System V and BSD flavors of Unix. That effort to create a single unified Unix version foundered in opposition from IBM and HP, who feared AT&T and Sun would have early access to the code and a competitive advantage in the U.nix market, but the result was still significant.
He has subsequently left Sun, but leaves behind a lasting legacy in Unix development.
All hail Bill!
An interview of Bill Joy by Eugene Eric Kim - The Joy of vi - can be read here http://www.mat.upm.es/~jcm/bill-joy--vi.html