Basically he brought Bayesian Networks to machine learning. He did a lot in their structure and in particular efficient/approximate inference on Bayesian Networks. He has been contributing brilliant ideas since the early 80s I think.

Search belief propagation, junction tree algo, markov blanket, belief propagation statistical physics. The last search showing his ideas finding use beyond where he introduced them. Whenever ideas keep showing uses in different places especially in something as well evinced as the thermodynamic/information link then you know you have done something profound.