Well, as the linked article says, he basically coined the term "Bayesian network" and developed much of the underlying theory (e.g., which kind of independencies it can/cannot capture). This was arguably a crucial step for many areas of modern AI in terms of dealing with uncertainty, cf. the full ACM press release: http://www.acm.org/press-room/news-releases/2012/turing-awar...

He is also credited with first describing the ever-popular belief propagation algorithm for approximate reasoning in Bayesian networks and graphical models in general.

Edit -- I found this to be a nice, short exposition: http://ftp.cs.ucla.edu/pub/stat_ser/r368.pdf